When finding the correct answer choice that strengthens the argument in this question, it should be noted that the answer must be directly relevant to the specific argument in question. Also, it must provide information that justifies the argument.

Premise: The union membership was higher in 2010 than in 2018.

Now, from the options provided, let's find the option which the premise directly and relevantly link to.

Option A - Incorrect
This is not true as we only know that the union membership was higher in 2010 than in 2008. The passage did not provide us with the report of the union membership in 2009, and we can't assume a stat for this.

Option B - Correct
This answer choice compliments the passage premise, and it gives a concise reason that strengthens the argument. In other words, the fact that the factor had more employees in 2010 than in 2008 validates the reason why the union membership was higher in 2010 than in 2008.

Option C - Incorrect
This choice of answer is too general as it talks about the revenue in dues generated by the union in 2010 and in 2008, which are irrelevant to the passage statement.

Option D - Incorrect
The statistics of the union membership in 2009 was not given, and as such, we cannot assume there was an increase in the union membership in that year.

Option E - Incorrect
We cannot validate this from the passage because there was no statistics given on non-union employees in the factory in 2010 and 2008 respectively. Aside from this, the passage is about union membership and not non-union membership.