So (a) If you double the diameter, does the circumference double? Yes. So you're right.
(b) If you double the average rate of travel (in other words, the speed) does the distance you'll travel in two hours double also? Answer: yes. So you're not right - this does vary directly.
(c) If you buy twice as much gas, does it cost twice as much? Yes. So you're right here.
(d) If the radius of a circle is doubled, does the area double? No, it goes up by a factor of 4, because it's proportional to the square of the radius. So, you're not right on this one.