Is each bioconditional statement true of

"A number is a multiple of 3 if and only if it is odd". False. A counterexample would be the number 5 which is odd but it's not a multiple of 3.

"A number is even if and only if the number is divisible by 2". True. This is the exact definition of what it means to be even. Another way to put it: if a number is a multiple of 2, then it is even and vice versa"

"A number is prime if an only if the number is not a multiple of 4". False. The number 9 is not a multiple of 7 and it's also not prime either.