Premises:
The average salary for jobs requiring a college degree is higher than the average salary for jobs that do not require a degree.
Over the last few years, the number of Kravonians enrolled in college has been growing steadily.

Conclusion:
The number of Kravonians entering the job market who have at least the qualification of a college degree will eventually be significantly higher than it has been over the last few years.

Note a few things here: The first sentence of the argument "average salary etc" is irrelevant to the argument. The argument could very well have omitted it and it would still be the same. It just gives a possible reason why more people are enrolling in college. The conclusion is that people who have college degrees will become significantly higher.

We need to weaken the argument.

(A) Kravonians with more than one college degree earn little more, on average, than do Kravonians with only one college degree

One college degree vs multiple degrees is certainly a promising premise but here is the problem - this option doesn't say that more people are enrolling for 2nd/3rd degrees or something like that. It says people with more degrees don't get more money. That has nothing to do with our argument. We are concerned about how many people are there who have at least one college degree - who gets more money who gets less is not relevant to the argument. Even if people with multiple degrees were getting much more money, we cannot assume that more people must be enrolling for multiple degrees. Hence, this option is irrelevant.

(B) The average number of years Kravonian college students remain enrolled before completing a degree has been increasing over the past several years.

More people have been enrolling in college, but on average the time they are spending in college is also increasing. If they keep spending more and more time as the years go by, it is unlikely that we will have "significantly higher" number entering the job market. More people will enrol year after year steadily but they will also keep spending more and more time there so the "significantly higher" number may not happen. The number of people entering job market with college degrees may also keep increasing steadily.

(C) Despite the increase in the number of Kravonians attending college, the percentage of the population attending college is lower in Kravonia than in most other countries.

Other countries' numbers are irrelevant.

(D) In recent years, employers have been requiring college degrees for workers in jobs that were previously performed successfully by Kravonians who did not have college degrees.

Again irrelevant. The demand for college degree workforce is increasing. It doesn't weaken the conclusion that there will be a significantly higher college educated workforce.

(E) For many years, employers in Kravonia have had difficulty finding enough college graduates to fill the high-paying jobs that were available.

Irrelevant. As discussed, what the job market wants is irrelevant to our argument.

Answer (B)