I just wanted to download TensorRT but I saw there are two different versions GA and RC. What is the differences between them and what version should I choose as I use a Windows 10 machine with Geforce GTX 960M graphic card?
GA is a production release, it stands for "General Availability". It is the version most people should be using, for most activities. It has gone through a full test cycle.
RC is a "Release Candidate". It is a preproduction version, an early release version. In general, the recommendation would be to use a GA version, unless you have a specific reason to use a RC version (e.g. to evaluate a new feature). RC might be used to evaluate a specific new capability before it is available in a GA release, or it might be used to evaluate compatibility of a current code or application with future plans.
Recommondation based on your question: Use the GA version.
Note that each release is numbered, and a numbered release or release "cycle" could have (first) a RC version followed later by a GA version.