In the US, One's SAT Score May Be Used as a "Proxy

In the U.S., one's SAT score may be used as a "proxy" measure of one's social class. What does this mean?

A) The SAT score is a more realistic measure of social class than income or education.
B) SAT scores are the best predictors of how well a student will do in their career.
C) Family income is strongly correlated with scores on the SAT exam.
D) SAT scores are more related to race than income.