Hello!
The answer is the first option, “Cuántos.”
Let’s first understand what the words in the sentence mean. Excluding the blank, the sentence means ____ years is he?
We can automatically cross out “su” and “mis” because they mean “his” and ”my.” Those words would make the sentence grammatically incorrect. Now, what’s the difference between “cuánto” and “cuántos”? Keep in mind that the word years is plural. Cuántos is a plural word, while cuánto is singular.
I hope this helped you!
~ Have an amazing day ~ :)