If a, b, c, l, m. n are in A.P., then 3a + 7, 3b + 7, 3c + 7, 3l + 7, 3m + 7, 3n + 7 form

Terms of an AP if multiplied with a constant will still be in AP.

The constant term is 3.

Therefore, 3a, 3b, 3c…are in AP.

Adding a constant to an AP does not change the sequence type. i.e. It will still be in AP.

The constant term added is 7.

Therefore, 3a + 7, 3b + 7….also form an AP.

The correct choice is B.