Pins and Skewers - Lesson 27

Hi All:

I hope somebody can help me with Lesson #27 of Pins & Skewers - Tactics. GM Wolff indicates Nxd4 as the correct move rather than N x Ra1. I am puzzled by this because the solution move leaves Black ahead by a bishop, but based on my thought of NxRa1, Black would be ahead by a Rook. Does anyone have any idea why the solution move is preferred?

It is a matter of material:

See below

Could anybody please tell me where these lessons are given?

Sreejish, you can go to http://www.chess.com/chessmentor/myhome.html to access the Chess Mentor program on Chess.com

tht black knight is trapped...Material advantage is too damage than ..having a bishop for a opponent rook