See explanation below.
Work Step by Step
This answer can be found in the chapter 24 reading. a.For 48 K19, N/Z ratio is 29/19 = 1.5, Too large therefore it is unstable. b.For 79 Br35,N/Z ratio is 44/35 = 1.3, moderate, therefore stable. c.For 32 Ar18,N/Z ratio is 14/18 = 0.78, too small, therefore unstable.