For necessary assumption questions, we always negate the answer choices that we are considering in order to make sure that when negated, these will hurt the argument.
In this case, the negation is as follows: "Using the phases of the moon does not usually lead amateur gardeners to plant later in the spring than those planting at the first warm spell".
Taking this new negated statement, this weakens the argument that amateur gardeners who use the phases of the moon are less likely to lose plants to a frost.
Answer choice B, however, does not weaken the argument when negated. It becomes "the phases of the moon do not affect whether a frost follows the first warm spell of spring".
This doesn't weaken the argument because it makes no difference if the phases of the moon actually have an impact on the frost occurring, but rather it just needs to somehow lead amateur farmers to avoid losing plants to a frost.