typedef long a;
extern int a c; while compiling this statement becomes extern int long c;. This will result in to "Declaration syntax error".

typedef long a;
extern a int c; while compiling this statement becomes extern long int c;. This will result in to "Too many types in declaration error".

typedef long a;
extern a c; while compiling this statement becomes extern long c;. This is a valid c declaration statement. It says variable c is long data type and defined in some other file or module.

So, Option C is the correct answer.