See explanation below.
Work Step by Step
a. For carbon-15,N/Z ratio is 9/6, it has more neutrons, so to decrease neutrons, it undergoes beta decay. b. For 120 Xe,N/Z ratio is 66/54, is proton rich.SO it undergoes either positron emission or electron capture to become stable. c.For Thorium-224, N/Z ratio is 134/90. Atomic number of Thorium is 83, so it is unstable and undergoes alpha decay to become stable.