using SOQL Query not able to fetch Account Applicant_ID__c

I have an external id in Account named Applicant_ID__c. I am using data loader to import data into salesforce Opportunity. Below is my mapping file content.

Date\ Cancelled=Date_Cancelled__c
Date\ Denied=Date_Denied__c
Date\ Decisioned=Date_Decisioned__c
App\ Status=Application_Status__c
Date\ Submitted=Application_Submitted__c
Closing\ Date=CloseDate
Application\ Source=Application_Source__c
Application\ Type=Application_Type__c
Application\ Sub-Type=Application_Sub_Type__c
App\ ID=App_ID__c
Property=Property_Name__r\:Property_Code__c
Applicant\ ID=Account\:Applicant_ID__c
Record\ Type\ ID=RecordTypeId

The above mapping is working correctly now what i want is to populate the opportunity name from trigger. Below is my trigger content

trigger MapStatusToStageBeforeOpportunintyCreation on Opportunity (before insert, before update) {

for (Opportunity o : Trigger.New){


Account acc = [Select LastName From Account Where Applicant_ID__c =:Account:Applicant_ID__c];

o.Name =acc.LastName;
}  

}

Thanks in advance.

That answer you created and excepted is going to blow up if insert 101 Opportunities, but if you want to use the Applicant_ID__c you can query it out in the account query

trigger MapStatusToStageBeforeOpportunintyCreation on Opportunity (before insert, before update) 
{
    Set<ID> acctIDS = new Set<ID>();

    for (Opportunity o : Trigger.new)
    {  
        if(o.AccountId != null)
        {
            acctIDS.add(o.AccountID);
        }                
    } 

    Map<ID, Account> acctMap = new Map<ID, Account>([Select LastName, Applicant_ID__c  From Account Where ID =: acctIDS]); 

    for(Opportunity o : Trigger.new)
    {
        for(ID acctID :acctMap.keySet())
        {
            if(o.AccountID == acctID)
            {
                o.Lastname = acctMap.get(acctID).LastName;          
            }
        }   
    }       
}