If John saved an average of $50 per month for the first and last months, then he saved a total of $100 in those months, and his savings in the second month must be $100. This statement is sufficient.
The correct answer would be between A/D.
If John saved x in the second and third months combined, then his savings in three months is (2/3)x + x = (5/3)x = 200. We can solve for x, the amount he saved in months two and three overall, but we don't know how much he saved in month two versus month three. This statement is insufficient.
The correct choice is A.