Hi Pavan
 try this code
 
 
 <apex:page controller="accountfiltercon">
    <apex:form >
        <apex:pageBlock title="Accounts">
            
            <h5>
                Account Name</h5>
            &nbsp;
            <apex:selectList size="1" value="{!selectedname}"> 
                <apex:selectOptions value="{!selectedaccnamefields}"/>
                <apex:actionSupport event="onchange" action="{!showOpps}" reRender="ListOpp">
                    <apex:param value="{!selectedname}" assignTo="{!AccName}" />
                </apex:actionSupport>
            </apex:selectList>
        </apex:pageBlock>
        <div class="slds-m-top_xx-large"></div>
        <apex:outputPanel id="ListOpp"  >
             <apex:pageMessages id="showmsg"></apex:pageMessages>
            <apex:pageBlock title="Account's opportunity" rendered="{!showHide}">
                <apex:pageBlockTable value="{!wrapperAccList}" var="opp">
                    <apex:column >
                        <apex:facet name="header">
                            <apex:inputCheckbox id="mainBox" value="{!mainBoxValue}" >
                                <apex:actionSupport event="onclick" action="{!checkUncheckAll}" reRender="ListOpp"/>
                            </apex:inputCheckbox>
                        </apex:facet>
                        <apex:inputCheckbox value="{!opp.flag}"/>
                    </apex:column>
                    <apex:column headerValue="Name" value="{!opp.oppObj.Name}"/>
                    <apex:column headerValue="Stage" value="{!opp.oppObj.StageName}"/>
                    <apex:column headerValue="Close Date" value="{!opp.oppObj.closeDate}"/>
                </apex:pageBlockTable>
            </apex:pageBlock>
        </apex:outputPanel>
    </apex:form>
</apex:page>

public class accountfiltercon {
    Public string selectedname{get;set;}
    Public String AccName {get;set;}
    Public List<Opportunity> oppList {get;set;}
    public boolean showHide {get;set;}
    public List<checkBoxWrapper> wrapperAccList{get;set;}
    public Boolean mainBoxValue {get;set;}
    
    Public List<Selectoption> getselectedaccnamefields(){
        
        List<Selectoption> lstnamesel = new List<selectoption>();
        lstnamesel.add(new selectOption('', '- None -'));
        for(Account acc :[SELECT id,name,phone,type,industry FROM Account]){
            lstnamesel.add(new selectoption(acc.id,acc.name));
        }
        return lstnamesel; 
    }
    public void showOpps(){
        System.debug('selectedname-->'+selectedname);
        if(selectedname!=null && (selectedname.length()==15 || selectedname.length()==18)){
            wrapperAccList = new List<checkBoxWrapper>();
            for(Opportunity opp :[select Name,StageName,CloseDate from Opportunity where AccountId=:selectedname]){
                wrapperAccList.add(new CheckBoxWrapper(false,opp));
            }
            if(wrapperAccList.size() > 0){
                showHide =true;
            }else{
                ApexPages.addmessage(new ApexPages.message(ApexPages.severity.FATAL,'This Account Opportunity Not avalaible'));
                
            }
            System.debug('oppList-->'+oppList);
        }
    }
    public void checkUncheckAll(){
        for(CheckBoxWrapper wrapObj : wrapperAccList){
            wrapObj.flag = mainBoxValue;
        }
    }
    public class CheckBoxWrapper{
        public Boolean flag{get;set;}
        public Opportunity oppObj{get;set;}
        public CheckBoxWrapper(Boolean flagValue,Opportunity oppObj){
            this.flag = flagValue;
            this.oppObj = oppObj;
        }
    }
}

if you found this answer helpful then please mark it as best answer so it can help others.

Thanks 
Ajay Dubedi