Answer
3 peaks
Work Step by Step
In the mass spectrum, we will see one peak for each type of molecule. Since the molecule we are looking at is $H_{2}$ and we are told that the conditions will prevent decomposition, we need to see how many combinations are possible between the two isotopes, 1H and 2H. We see that we can have 1H-1H, 2H-2H, 1H-2H, 2H-1H but that 2H-1H and 1H-2H would have the same mass and therefore the same peak. This leaves us with 3 possible arrangements so 3 peaks .
