What is the best tool to maintain the traceablity.

Question

The tool which can be used to trace requirements specifications, design documents, source code, test plans.

Answer 1

The best tool in the world won't help you trace requirements like this unless you have a good and stable process in place to deal with these types of things.

Something like this is dependent on the people using the tool to all be on the same page with their process, how they number requirements, etc.

Your tool could be as simple as "give each requirement a unique number and make sure that it is referenced with every check-in to the file repository". What you really need to worry about is the process, not the tool.

Answer 2

I used Rational RequisitePro and SoDA some time ago, it was pretty good.