This is why English is one of the hardest languages to learn!

Don't forget, you can use "AN historic event"
But "A historic event" seems to be correct too.

Here's some interesting info:

You should use “an” before a word beginning with an “H” only if the “H” is not pronounced: “an honest effort”; it’s properly “a historic event” though many sophisticated speakers somehow prefer the sound of “an historic,” so that version is not likely to get you into any real trouble.

https://www.wsu.edu/~brians/errors/anhistoric.html

Same story with European. But I am not quite sure whether GMAT tests this concept but it does not hurt to know!

i have neva seen in my prep "vowels and article" being tested...