The correct option is D 14
28 = 1 × 28
28 = 2 × 14
28 = 4 × 7
So, the factors of 28 are 1, 2, 4, 7, 14 and 28.
42 = 1 × 42
42 = 2 × 21
42 = 3 × 14
42 = 6 × 7
So, the factors of 42 are 1, 2, 3, 6, 7, 14, 21 and 42.
Therefore, the highest common factor of 28 and 42 is 14.