Q. Why is there a compile time exception at 2 ?
R. Because of the  that is before the a4 identifier. The first part of the line says that you're declaring int arrays, the first one being named "a3". Then, after the coma, you must have only identifiers, as following:
int a3, a4;
Q. Is a5 a 1-D array & a6 a 2-D array ?
R. Yes. a5 is an int, and a6 is an array of int.