All linear functions can be written into the form y = mx+b
m = slope
b = y intercept
Choice D is of that form. In this case, m = 4 and b = -8
Choices A and C involve exponents larger than 1, which is why we don't have linear equations here. If you were to graph these, you'll find they produce nonlinear curves (i.e. curves that aren't straight lines). The same goes for choice B as well. Linear equations cannot have x in the denominator.
