How many utf$-8$ encoded characters can fit inside a "block" of AES?

Recall: each AES block is $128$ bits long.

Select one:

a$.$ always exactly $16$ characters.

b$.$ between $16$ to $65536$ characters

c$.$ It depends on which key length is chosen

d$.$ between $4$ to $16$ characters

e$.2^{128}$ characters