Answer:

(a) {X | X ∈ N, -100 ≤ X ≤ 100}

(b) { X | X ∈ multiples of 2, 2 ≤ X ≤ +∞ }

(c) { X : X is odd multiples of ± π/2, -nπ/2 ≤ X ≤ nπ/2}