The Morris Water Maze Is Used to Examine Spatial Learning

The Morris water maze is used to examine spatial learning in animals. Mice, who are good swimmers, are placed in a large pool of water and must learn to find a clear platform located just under the water surface. The platform is their only escape from the pool. If they do not find the platform, they have to continue swimming until we take them out.
We want to report an effect size estimate that reflects the fact that the animals were able to find the platform faster on the 10^th trial that they did on the first trial. What would be the best measure to report?

A) the difference in mean times on the two trials in question
B) the standardized difference in mean times
C) the correlation between performance on the first and tenth trials
D) the squared correlation between the two trials