The first thing to note is that addition of hydrogens to a molecule is usually a good indication that it is being reduced (this is something that is important to remember for the biologically relevant oxidation reduction reactions). Thus, going from NAD+ to NADH is a reduction reaction based on this principle. 

From there, the other part is to remember that reduction means GAIN OF ELECTIONS. If you have trouble with this, then use OIL RIG (Oxidation Is Loss, Reduction Is Gain). Thus, since we know that NAD+ is being reduced, it must be gaining electrons. The last part of this requires you to know the reaction for the reduction of NAD+, which isn’t as bad once you think about it. 

NAD+ is going to NADH. That means that NAD+ is going to add on an H+, and also be completely neutral. For this to happen you need two electrons! The first electron goes to make the NAD+ to NAD, and the second electron is added to “neutralize” the addition of the H+. If you didn’t have the second electron then instead of going from NAD+ to NADH, you’d go from NAD+ to NADH+. For this reason, the NAD+ gains two electrons and thus, the answer is B.