The word Perilous refers to extremely risky, dangerous and hazardous.

• Inane: The word inane means "not sensible," "lack of sense," "foolish," "being fooled by anyone," "nonsense," "dumb," and so on.
• Useless: The term useless refers to someone who does nothing, does not work properly, is not an achiever, is fragile, unsuccessful, and so on.
• Risky: In this context, the word risky has the same meaning as extremely dangerous, hazardous, something bad and risky, and so on, because it may be a related and appropriate account of the given question. So, it the right answer.
• Frivolous: The term frivolous means "one who lives with no purpose; aim or value, one who doesn't have any target or goal to realise, doesn't care, isn't serious, etc." 

Final Answer:

Hence, option C is the correct answer.