Explanation:
the expression is
45 × 12 + 45 × 32
this is the same as
45 × (12 + 32)
you can extract a common factor from a sum and make it the central factor for the sum.
it is the opposite operation of simplifying this central multiplication, where you have to multiply every part of the left factor with every part of the right factor and sin things up according to the resulting signs.
so, the 4th answer is correct.