The correct answer is A. Serious.
Trivial means not important or serious. It is something that is not worth thinking about or worrying about.
Serious means important or significant. It is something that is worth thinking about or worrying about.
Intricate means complex or complicated. It is something that is difficult to understand or do.
Calm means peaceful or quiet. It is something that is not noisy or exciting.
Dainty means delicate or pretty. It is something that is small and attractive.
In this case, the opposite of trivial is serious. A serious matter is one that is important or significant. It is something that is worth thinking about or worrying about. A trivial matter, on the other hand, is one that is not important or serious. It is something that is not worth thinking about or worrying about.