As you'll see in questions 199 & 200, inter-thermal speed is
independent of wind direction when on a XC flight. Headwind only
impacts speed to fly when flying to a goal on the ground outside the
moving reference frame of the airmass. You are correct and there is a
mistake in the quiz answer.

wrote:
80) What is the approximate proper airspeed to use when flying between
thermals on a cross-country flight against a headwind?

a) The best lift/drag speed increased by one-half the estimated wind
velocity.
b) The minimum sink speed increased by one-half the estimated wind
velocity.
c) The best lift/drag speed with no regard to wind velocity.

The SAA safety site gives the correct answer as "A". I have alway
heard that when flying between clouds you did not consider the wind in
your calculations. Maybe the tricky part of this question is the word
"Thermals".