This is like asking: What is the air-speed velocity of an unladen
swallow?

There are many more questions to ask before you can give an answer.
All the answers are wrong that are given on the exam. But there are
many more factors that would need to be answered:

1. Is this an intermediate leg or the final leg?
2. How strong is the average thermal?
3. If it is the final leg how close am I to final glide?
4. How strong is the wind?
5. Is there a wind gradient?

If this was meant to assume an intermediate leg, you need to climb back
to altitude at the end of the glide, no appreciable wind gradient, and
the wind is less than speed to fly then the correct answer is the
MacCready speed for the next expected thermal.

Tim