First Principles Thinking: forward payoff
B is correct. The CFA Curriculum states that a long forward payoff at maturity equals $S_T - F_0(T)$. Here, the payoff per barrel is USD 58.50 minus USD 64.00, which equals USD -5.50. For 1,000 barrels, the total payoff is USD -5,500. Therefore, the investor has a loss of USD 5,500.
Why option A is wrong: it uses the correct size of the price difference but reverses the sign, which would apply to the short side rather than the long side.
Why option C is wrong: it ignores the payoff rule and treats the forward price itself as a gain.