In this problem, we have two soft-drink cans -- the old can and the new can.
The old can is the same height as the new can. Let's call the height 1.
The new can has half the radius of the old can.
Let's call the radius of the old can 2 and the radius of the new can 1.
The new can costs 75% of the old can. Let's call the old price 4 and the new price 3.

Now, let's calculate volume, and then price per volume for both:

Old can:
Volume = pi*r^2 * h = pi * 4 * 1 = 4pi
Price = 4
Price per volume = 4/4pi = 1/pi

New can:
Volume = pi * r^2 * h = pi * 1 * 1 = pi
Price = 3
Price per volume = 3/pi

Percent change =
( 3/pi - 1/pi ) / (1/pi)
2/pi / 1/pi
2/1
2 or 200% (D)


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