15) F

The question clearly says that Uranium-238 is bombarded by neutrons. So the 1st stage would involve absorption of a neutron.

This would cause the mass number to increase from 238 to 239, forming Plutonium-239.

The following stages must involve ß-decay as the atomic number increases from 92 and 94. In each ß-decay, the atomic number increases by 1. So 2 ß-decays would allow the atomic number to increase from 92 to 94.