I don't thnk you can use 'c' as that's a local variable to the constructor.  You'll need to store the value in the controller somewhere.  Something like:

Public Class Comments{
public Comments__c Com{get; set;}
private Comments__c comWithRelated;

id str;
id str1;

public Comments(ApexPages.StandardController controller) {
Com=(Comments__c)controller.getRecord();
str=Apexpages.currentpage().getparameters().get('id');
System.debug('======================================>'+str);
}
public Comments(){
List<Comments__c> c=[select id, Contact__r.Id ,Finance__r.id, Application__r.Id from comments__c where id=:str];
if(!cisEmpty()){
for(Integer i=0;i<=c.size();i++){
str1=c[i].Contact__r.Id;
system.Debug('--->'+str1);
comWithRelated=c[0];
}
}

}

public PageReference lead() {
PageReference pageRef = new PageReference('/00Q/o');
return pageRef;
}
public PageReference Applicant() {
PageReference pageRef = new PageReference('/'+comWithRelated.Contact__r.id);
return pageRef;
}
public PageReference Application() {
PageReference pageRef = new PageReference('/'+comWithRelated..Application__r.id);
return pageRef;
}
public PageReference Finance() {
PageReference pageRef = new PageReference('/'comWithRelated.Finance__r.id);
return pageRef;
}
}

 Ideally these methods would check whether comWithRelated is null first, but I've left that as an exercise for the avid student.