when only two sides of a triangle are given
the minimum value of area is zero and
the maximum area is half of the product of the two given sides.
so in our case the minimum area would be zero and the maximum would be

(1/2) x 8 x 5 = 20.

hence D is the correct answer.

[As per Trigonometry area of a triangle is given as

= (1/2) abSin(x)
where a and b are the lengths of the sides of a triangle and (x) is the angle between them.
so area is minimum and equal to zero when (x) is zero degree. and
the area is maximum = (1/2) x 8 x 5 = 20 when (x) = 90 degrees]

let me know if you are satisfied

Let's avoid trig, since the GMAT doesn't expect us to know it.

Visualizing the triangle, we can make the area about as small as we'd like by simply extending the third side as far as it can go:



As our base approaches 8 + 5, our height approaches 0, so any area slightly north of 0 is possible. That means our lower bound is 0: we can't have 0, but we can have anything a hair away from it.

The upper bound is straightforward enough: we'll get it when the sides we have (5 and 8) are the legs of a right triangle. We can visualize this too:



See how the height (the orange line) shrinks when we bent the blue and red legs off a right angle?

That gives us a maximum of blue = height, red = base, or 5 * 8 / 2, or 20.

Summing up, we're left with 0 < area ≤ 20, and from there the problem solves itself.

Also, for those flashcarding this:

Given two legs of a triangle, the area of the triangle has the range: 0 < area ≤ (given side * other given side) / 2