2+2

4

Notes:

• In standard base-10 arithmetic, 2 + 2 = 4.
• In base-3, the sum is 11_3 (since 4 decimal = 11 in base 3).
• In bases ≥ 5, it's the single digit 4.
• The digit 2 is not allowed in base 2, so that base isn’t applicable here.