According to me, the answer is C.
The question says that 25% of the employees did not take a vacation. This can be interpreted as 75% took a vacation.
So the question involves a two-variable Venn diagram.
Statement - 1 : 1 in every 3 employees who took a vacation got a promotion. So this means that people who got a promotion as well as took a vacation is 25% of the total. However, there is no information about the people who got only a promotion as well as people who got neither of the two. So, INSUFFICENT.
Statement - 2 : The people who took a vacation but did not get a promotion is 500% of the number who neither took a vacation nor got a promotion. This statement is clearly INSUFFICIENT.
Combining both the statements.
The people who took a vacation as well as got a promotion is 25%. Hence, the people who took a vacation but did not get a promotion is 50%. This means that the people who neither took a vacation nor got a promotion is 10% of the total. Thus, the people who got a promotion but did not take a vacation is (100-50-25-10)%, i.e. 15%. This means that the number of people who got a promotion is 35% of the total.
Thus, both statements considered together are SUFFICIENT.