Which of the following struct definitions is correct in C$++?$

a$.$ int struct studentType $\{$ int ID; $\}$

b$.$ struct studentType $\{$ int ID; $\}$;

c$.$ struct studentType $\{$ ID; $\}$;$$

d$.$ struct studentType

$\{$

string name;

int ID;

double gpa; $\}$