Why sizeof int is wrong, while sizeof(int) is right?

Question

We know that sizeof is an operator used for calculating the size of any datatype and expression, and when the operand is an expression, the parentheses can be o

Answer 1

From C99 Standard

6.5.3.4.2
The sizeof operator yields the size (in bytes) of its operand, which may be an expression or the parenthesized name of a type.

In your case int is neither expression nor parenthesized name.