Fill in the blank with the most appropriate option:

Mounting unemployment is the most serious and ______ problem in the country. 

Intractable 

The most appropriate word to fill in the blank is intractable.

The other options are not as accurate. Dubious suggests that the problem is doubtful or uncertain, which is not the case. Mild suggests that the problem is not very serious, which is also not the case. Unpopular suggests that the problem is not liked or popular, which is not the main issue. Intractable is the best option because it suggests that the problem is difficult to control or deal with. This is the most accurate description of the problem of mounting unemployment, as it is a complex issue with no easy solutions.