A factor is an integer multiplied with another integer to get the product.
Factors of 32 are 1, 2, 4, 8, 16, 32. Among the given choices, 4 is the factor of 32.
Factors of 12 are 1, 2, 3, 4, 6, 12
Factors of 21 are 1, 3, 7, 21
The common factors of 12 and 21 are 1 and 3. Among the given choices, 3 is the common factor of 12 and 21.
