The missing item that would complete the alpha decay reaction is 232/90 Th. Option B is correct.
The missing item that would complete the alpha decay reaction is option B) 232/90 Th. In an alpha decay reaction, an alpha particle (which is a helium-4 nucleus, 4/2 He) is emitted from the parent nucleus, resulting in the formation of a new element and an alpha particle.
In this case, the parent nucleus is 228/88 Ra, and the missing item must combine with the alpha particle to form the new element. Among the options given, only 232/90 Th satisfies this requirement, as it combines with 4/2 He to form 228/88 Ra.
Hence, B. is the correct option.