Answer: Carlos es un chico mu simpático.
Explanation: The sentence that we have is:
"Carlos es un chico muy _____"
Here the subject is Carlos, that is a name for a male and is a singular subject.
The options are:
A) Ordenada: (as most adjetives finished in -a, this is a feminine adjetive, so this is incorrect)
B) Gusto ( This is not an adjetive)
C) Estudiosos (this ends in -s, which means that this is a plural adjetive, so this is incorrect)
D) Simpático (This is singular and masculine, so this is the only option that makes sense)
Carlos es un chico mu simpático.