The function used to convert a string to an int is . . . . . . . .

Answer: Option D

INET_ATON(): This function is used to convert an IP address in dotted-quad notation (IPv4) to a numeric value. It returns the numeric representation of the IP address as an unsigned integer.
INET_NTOA(): This function is used to convert a numeric value representing an IP address to its dotted-quad notation (IPv4). It returns the string representation of the IP address.
INET_ITOS(): This option is incorrect. There is no function named INET_ITOS() in MySQL for converting a string to an integer.
INET_STOI(): This option is incorrect. There is no function named INET_STOI() in MySQL for converting a string to an integer.

Therefore, the function used to convert a string to an integer in MySQL is Option D: INET_STOI().