The equations are made to confuse you.
Helium has a mass number of 4 because it has 2 protons and 2 neutrons.
We are given the information that 4 protons are used to make Helium.
Since a Helium atom has 2 protons and 2 neutrons, 2 out of the original 4 protons would have to be converted to 2 neutrons.
Therefore, 2p -> 2n + 2e+
Hence, B is the answer.