A.
struct {long int num; char name[20]; char sex; char addr[20]; }student; struct student std1;
B.
#define STUDENT struct student STUDENT {long int num; char name[20]; char sex; char addr[20]; } std1;
C.
struct {long int num; char name[20]; char sex; char addr[20]; } std1;
D.
struct student {long int num; char name[20]; char sex; char addr[20]; } std1;