One thought (performance is not in my speciality, so I may have the wrong end of things) is that for level, unaccelerated flight thrust and drag are equal, so max drag in level flight = max thrust. Max drag for level flight is therefore assumed constant (as thrust available is assumed constant), so at max level speed drag is a maximum, as the aircraft will accelerate until this maximum is reached. Considering how the graphs of thrust available and thrust required/drag against EAS are drawn, i.e. for level flight, this may be the consideration the examiners are making.
I see the question as reported here to be ambiguous (it does not say level flight). The level of debate, even among instructors, suggests it is a poor question.