In stars four protons, p, are fused together to form helium, He, with a mass of four units, by the following processes. Which one of the following shows the overall change that has taken place when He is formed? (e^+ is a positively charged electron, n is a neutron, H is a hydrogen of mass two and He is helium of mass three.)

Answer:

25) B

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.