To have a real component of 4, the complex solution must have the form 4 + bi, where b is the imaginary component.
We can use this fact to eliminate options A and C, which do not have a complex component at all.
Option B is also incorrect because it simplifies to 22 + 8x + 16 = -21, which gives a real solution of x = -5/4, but not a complex solution with a real component of 4.
The correct option is D, which gives the equation (2+4i)/(8+16i) = 4, which simplifies to 2+4i = 32+0i. This equation has a complex solution with a real component of 4, as 4 is the real component of 32+0i.
Therefore, the correct answer is option D: 2² / (8x16 -21) = 4.