Final answer:
The circumference of a circle is directly proportional to its radius, but the area is not.
Step-by-step explanation:
The circumference of a circle is directly proportional to its radius, meaning that as the radius increases, the circumference also increases. So, option a is correct. On the other hand, the area of a circle is directly proportional to the square of its radius. This means that as the radius increases, the area also increases, but at a faster rate. Therefore, neither of the given options is correct regarding the area.