The answer is not entirely clear-cut and depends on a few factors such as temperature, wind, and initial relative humidity before the rain started. However, in general, when it rains, the relative humidity tends to increase. This is because rainwater is evaporated into the air, which increases the amount of water vapor in the air.

However, students have reported that the correct option in their exam was a) "Stays at 100%", this could be because the question assumed that the initial relative humidity was already at 100% before the rain started. In that case, if the amount of water vapor in the air increased due to the rain, it would reach its saturation point, and the relative humidity would remain at 100%.

So, it's possible that the question was looking for a specific answer based on specific assumptions, but in general, when it rains, the relative humidity tends to increase.