Answer : 4. "serious"
Explanation :

The most appropriate antonym for "trivial" in the given sentence is "serious." "Trivial" refers to something of little importance or significance, so the opposite would be something of great importance or seriousness. The other options— "underrated" (underestimated), "kind" (having a gentle nature), and "problematic" (posing a problem or difficulty)— do not convey the opposite meaning of "trivial" in this context.