What is the smallest digit that can replace ☐ so that 7☐45 is divisible by 3?

Explanation

1. 01

   A number is divisible by 3 when its digit sum is a multiple of 3.

2. 02

   $7 + 4 + 5 = 16$; the next multiple of 3 is 18.

3. 03

   The smallest digit is $18 - 16 = 2$.