If A

Given matrix orders

[A] with 3 x 4 and [B] with 4 x 3

We have to find the order of [B].[A]

For matrix multiplication the columns of 1^st matrix and the rows of 2^nd matrix should be equal.

E.g. [A]_3x2.[B]_2x2

The order of new matrix formed after multiplication of matrices will be 1^st matrix row will be taken as new matrix rows. The no. of columns in 2^nd matrix will be taken as new matrix columns.

E.g. [A]_3x2.[B]_2x2 = [AB]_3x2

In the same way [B]_4x3.[A]_3x4 = [AB]_4x4

∴ option (B) is correct answer