In binary the one on the left is meaningless, and therefore the two cannot be compared. In any base in which they can be compared, the one on the left is smaller.
Obviously he is correct because the smallest base that can represent 10 is base 2 and 10 in base 2 is equal to 2 in base 10. And the smallest base in which you can represent the number 3 is base 4 and 3 in base 10 in equal to 3 so 2 is the smaller number hence "10" is the smaller number. And from the drawing of the rainbow you can infer that he wants to use a diverse range of bases and not just the common base 10.
Btw I am only talking about the natural bases (whole number positive).
What are you supposed to write there? I guess 3 < 10 is not the answer. It also requires text, so drawing 3 vs 10 of something isn't suitable, too. "You taught us" or what do they want to hear??