Beta particles are fast moving streams of electron, having a mass number of 0 and atomic number as -1. If Thorium undergoes radioactivity to emit a beta particle, it means mass number of the new element Pa will be unchanged but the atomic number will be increased by 1. i.e 91. Thus, y = 234-0 = 234, while x = 90 - (-1) = 90 +1 = 91. Option A is the correct answer