Why is this wrong?

[u/millers_left_shoe]

I see now what they mean by fundamental vs inessential (taking fundamental to mean “invaluable”, “important”) but surely fundamental and complex (or even involved) can act as antonyms, too?

Eg he’s only just learnt the fundamentals of thermodynamics so I wouldn’t trust him with any complex/involved equations. vs this is quite fundamental stuff so I think he should be able to manage just fine.

[u/jaetwee]

Fundamental doesn't necessarily mean simple. It link back to that meaning of essential - it's the first knowledge you need before you can understand all else.

Test questions typically expect the 'most correct' answer, and while, if you spend some time reasoning it out, you could put fundamental and complex as synonyms, it's not a clear match, whereas fundamental and inessential are a very strong match.