In the question, I have heard that they had asked for pumping length.
Answer will be 24.
But it is not the minimum pumping length. So I guess they just asked about pumping Length(which is any number for which pumping lemma condition is satisfied by the language) for this language.
Say P is the minimum Pumping length for a language..then every number greater than or equal to P is a Pumping Length for the language.
You could have simply eliminate the other three options by seeing that the minimal string in b is $b^{10}$ in the language. Now if pumping Length were less than 10 then when we pump any part to power 0 of this string $b^{10}$.. then it doesn't belong to the language.
You didn't even need to solve this question if you knew the fact that " if P is a Pumping length for a language..then every number greater than or equal to P is a Pumping Length for the language."
I'll give full detailed explanation later when GATE 2019 Questions will be uploaded on GO.
Answer is 24.