Which pair of complex factors results in a real-number product?

a. 15 (-15i)

b. 3i (1-3i)

c. (8 + 20i) (-8 - 20i)

d. (4+7i) (4-7i)

To answer the problem above, we must first define what "i" means. i means imaginary number which means square root of - 1, to get rid of i, you simply need to square, in this problem, we need to look for an equation where "i" is squared. The answer is D. (4+7i) (4-7i) to justify:

(4+7i) (4-7i)

(16-28i+28i-49i^2)

(16-49i^2)

16-49 = - 33