It is given that 90 percent of a club’s performance can be explained by wage bills. If a player’s salary is based on only on their abilities then the clubs that paid more amounts must finish higher. This relation will not hold true if there is pay discrimination. The conclusion of the passage is that certain clubs seem to have achieved less than they could have, by not recruiting black players.
Here, the assumption is that certain clubs paid more than many other clubs but did not finish at a position they could have had.
Now, let’s check each option and verify whether it strengthens Szymanski’s conclusions.
Option A is not relevant here because it does not address our assumption in any way. Hence, option A is wrong.
The clubs that the author is talking about could have chosen a few black players, paid the black players lower wages and achieved as much as they did now. So, if it is a fact that clubs hired white players at relatively high wages and did not show proportionately good performance, then we can conclude that these clubs achieved less than they could have had. Hence, option B is correct.
According to option C, the towns have a history of discrimination against blacks. But, the clubs might or might not show the discrimination. Hence, the underperformance of these clubs cannot strengthen Szymanski’s conclusions. Therefore, option C is wrong.
Option D is a fact which can be deduced from the information given in the passage. Hence, it doesn’t strengthen the conclusion.
Therefore, option B is the correct answer.