Concept:The word “trivial” means something small or unimportant, so its opposite should mean important or weighty.
Explanation:A trivial issue is of little value or seriousness.
When it is not properly handled, it grows into a more important or severe matter.
Among the options, only “serious” correctly expresses this opposite meaning.
“Valuable” refers to worth, not severity.
“Ordinary” is close to trivial, not opposite.
“Unbearable” is extreme and does not fit the natural contrast.
Answer:A. serious