(A) The economy slowed, thus creating a decreased demand for college graduates.
Doesn't resolve, this option further deepens the discrepancy, we know that supply of college graduates remained unchanged, if demand decreased then it should have been a more punitive for college graduates' salary as compared to salary of only high school diplomas.
(B) The quality of high school education improved.
Doesn't resolve, this option further deepens the discrepancy, this should have led to decrease in gap of salaries between college graduates and high school diplomas.
(C) Compared to the 1970’s, a greater number of high schools offered vocational guidance programs for their students.
Doesn't resolve, what happened with vocational guidance program doesn't help us to resolve the increased salary gap between college graduates and high school diplomas.
(D) The proportion of the population with at least a college-level education increased.
Doesn't resolve, knowing about the proportion doesn't help us to resolve the increased salary gap between college graduates and high school diplomas.
(E) There was for the first time in 20 years an oversupply of job seekers with only high school diplomas.
Resolves, so what actually happened was decrease in salary of high school diplomas was due to their over-supply.
We can understand using a short example:
Let's say salary of a college graduate in 1970 was $100 which let's assume was also same in 1980
Salary of a high school in 1970 was $82 which fell to $53
We can see that we can't blame college graduates for this increased gap, rather it is due to high school diplomas only.