All of them are Lexical Errors. Lexical analysis is implemented using Finite Automata.
Option A: int a b;
Start->int->a->"Expected either ; or = but got whitespace." (Error) Generated Tokens: 2
Option B: in t a b;
Start->i->n->"Expected t but got whitespace" (Error) Generated Tokens: 0
Option C: int 1ab2;
Start->int->"Expected a permitted character but got 1"->a "Didn't expect a character after a number" (Error) Generated Tokens: 1
Explanation of Option C: A parser doesn't support Backtracking. After getting 1 (Which was not permitted) the compiler got a which means that it has to backtrack.