Let's start with answer choice C. This answer choice states that maximizing training for long-distance running requires alternating between the two strategies. There are two issues with this answer choice and we will examine both. Firstly, this is not an deduction we can make, nor is it supported anywhere in the passage. Based on this passage alone, we do not know what it takes to maximize the benefits of training for long distance running. Perhaps, a runner should stick to one strategy and become masterful at employing it. Or rather, instead of alternating frequently, a runner should alternate rarely. We simply do not know.
The other issue is that the question is asking us to complete the argument. Imagine now that we apply this answer choice to fill in the blank. This will become, "Since it is important for long-distance runners to enter a race mentally refreshed, maximizing the benefits of training for long-distance running involves frequently alternating associative and dissociative strategies." This doesn't seem to be the direction that the passage was heading. Considering the premises discussed in the passage were concerned with mental exhaustion, this should hint to us that the correct answer choice will have something to do with this as well.
This is exactly what we find in answer choice A, which states "long-distance runners should not rely heavily on associative strategies during training the day before they run in a race". This follows logically because if they did do that, then they would be mentally exhausted and this would hinder their results.
Hope this helps, please let me know if you have any other questions.