//apex controller 

public class Forum_TestClassCorrection {
	public class fourlists {
        public List<Integer> firstlist ;
        public List<Account> secondlist = new List<Account>();
        public List<Opportunity> thirdlist ;
        public List<String> fourthlist ;
    }
    
    public List<Account> selected (List<fourlists> FL){
        return FL[0].secondlist;
    }  
}

//test class 
@isTest
public class Forum_testInnerclass4List {

    static testMethod void testmethodfourlist(){
        //insert test account
        Account acct = new Account();
        acct.Name = 'Ashok';
        acct.AnnualRevenue = 10000;
        acct.Phone = '0731467890';
        insert acct;
        
        //ceate list of instances of wrapper class 
        List<Forum_TestClassCorrection.fourlists> listInstance = new List<Forum_TestClassCorrection.fourlists>();
        
        //create instance of wrapper class 
        Forum_TestClassCorrection.fourlists wrapperInstance = new Forum_TestClassCorrection.fourlists();
        
        //insert account in the wrapper instance 
        wrapperInstance.secondlist.add(acct);
        
        //insert wrapper instance in wrapper list
        listInstance.add(wrapperInstance);
        
        Test.startTest();
        //call the class 
        Forum_TestClassCorrection FT = new Forum_TestClassCorrection();
        //call specific method
        FT.selected(listInstance);
        Test.stopTest();
        
        
        
        
    }
        
}