Answer:
A. Video Card
Explanation:
Assuming that he has purchased everything as such. A video card or graphics card is the key thing he's missing to play any games and being VR-compatible. B and D are not really necessary to gaming and although having an C is kind of needed. Playing on an old monitor is fine as well, just as long as it has an HDMI or DP input. All that's left is the video card.