According to Modern Physics pages 488 and 489, the proton-proton chain is believed to be responsible for most the sun's energy. The proton-proton chain involves three different reactions

$\begin{align}^1H + {^1 H} &\to {^2H} + \beta^+ + \nu \mbox{\ \ \ \ \ \ \ \ (Q = 0.42 MeV)} \label{eqn19:1}\\^2H + {^1 H}...$
The reactions with labels (1) and (2) must each occur twice. Therefore, 6 $^1H$ s are required to produce 2 $^1H$ s along with an ${^4He}$ . Therefore, the proton-proton chain results in a net loss of 4 $^1H$ into a net gain of ${^4He}$ . Therefore, answer (B) is correct.

Solution to 2001 Problem 19