<apex:page standardController="Lead" extensions="opentasks" >
    <apex:form >
        <apex:pageBlock title="open Tasks" mode="edit" id="opnTsks"> 
            <apex:Messages />         
            <center><a href="https://ap1.salesforce.com/00T/e?who_id={!leadld}&retURL={!leadld}" target="_top"><button type="button">New Task</button></a>
                <apex:commandButton value="close" action="{!close}" rerender="opnTsks"/>
            </center>&nbsp;&nbsp;
            <apex:pageBlockTable value="{!listwrapper}" var="each" >
                <apex:column headerValue="Action">
                     <apex:inputCheckbox value="{!each.checked}"/>
                    &nbsp;|&nbsp;
                    <apex:outputlink value="/{!each.k.id}/e?retURL={!each.k.WhoId}" target="_top"> Edit </apex:outputlink>
                   </apex:column>
                <apex:column headerValue="Subject" value="{!each.k.Subject}"/>
                <apex:column headerValue="ActiveDate" value="{!each.k.ActivityDate}"/>
                <apex:column headerValue="Status" value="{!each.k.Status}"/>
            </apex:pageBlockTable>
        </apex:pageBlock>
     </apex:form> 
</apex:page>

public class opentasks {
    public List<OpenActivity> open{get;set;}
    public Id leadld{get;set;}
    public list<wrapperclass> listwrapper{get;set;}
   
    public opentasks(ApexPages.StandardController controller){
        leadld = ApexPages.CurrentPage().getparameters().get('id');
        list<task> open1 =new list<Task>(); 
        listwrapper = new list<wrapperclass>();
        list<lead>   le =[SELECT Name, (Select Id,Subject, WhoId, ActivityDate,Status, Priority, OwnerId FROM Tasks WHERE IsClosed=false) 
                            FROM Lead WHERE Id = :leadld];
        if(!le.isEmpty()){
            for(Task k:le[0].Tasks)
            {
                listwrapper.add(new Wrapperclass(k));
            }
        }
    }
    
    public class wrapperclass
    {
        public  boolean checked{get;set;}
        public Task k{get;set;}
        public Wrapperclass(Task k){
            this.k=k;
        }
    }
    public PageReference close()
    {   
        list<Task> listofopen=new list<Task>();
        if(!listwrapper.isEmpty()){
            for(Integer i=0; i < listwrapper.size();i++)
            {
                wrapperclass w = listwrapper[i];
                if(w.checked==true){
                    system.debug(w);
                    w.k.status='completed';
                    listofopen.add(w.k);
                    listwrapper.remove(i);
                }
            
            }
        }
        if(listofopen.isEmpty())
        {
            ApexPages.addMessage(new ApexPages.Message(ApexPages.Severity.ERROR,'Select atleast one column t'));
        }
        else{
            update listofopen;    
        }
        return null;
    }   
 }