Actually, your "correct answer" isn't correct. It should be Райн хорошо выучиЛ новые слова, потому что он учил их почти час. Ryan has learned new words well because he had been learning them for almost an hour. or Райн хорошо выучиТ новые слова, потому что он БУДЕТ УЧИТЬ их почти час. Ryan will have learned new words well because he will be learning them for almost an hour.

You can't say "выучил их почти час", because "почти час" indicates continuity (he did / was doing smth for an hour), and you have to use the imperfective verb to express continuous actions. You can say "он учил их почти чаc" (continuous action), or "он выучил их час за один час" (completed action), or "он выучил их час назад" (completed action).

I think, the correct answer is not entirely correct. It means: "He WILL learn them well because he learnED them for an hour." The first part clearly means that he'll complete it in the future. It makes sense if he is going to spend one more hour, learning them in 2 sessions. It is a guess about the future: "I think, he will learn them well, I think so, because he..." It's correct but complex. Do they expect you to reconstruct this context: a learner learning vocabulary two evenings, a person guessing the outcome between the two evenings? Likely they mean this: Райан хорошо выучиЛ новые слова, потому что он учил их почти час. "He learned them well, because he learned them for an hour / spent an hour learning them." Here "learned well" can be a fact, not a guess. The second part explains how he managed to do that. Another one: Райан хорошо выучиТ новые слова, потому что он учиТ их почти час. "is-learning them almost hour" informs you that he's been learning them for almost a hour and is still learning. Present because he is still doing this. Actually, a Russian is tempted to add уже (already) to point at "have been" meaning and to exclude "he [usually] learns them for an hour" reading. A guess about the future, but this one is less complex: he'll complete it in future, at the moment he's learning. Given the time already invested I think WHEN it will be completed, he'll remember them well. P.S. I wrote the above yesterday, I just wanted to make it reasonably short:) Since then other people told the same. I send it anyway, hope it won't harm.

Think of English "to learn" as two verbs. Learn-1 is выучить. It is like in "I finally learned them well". You have them in your memory now, it's an accomplishment. Learn-2 is учить. It is like "I walked around for 2 hours". It is about how you spend time. "I've been learning them for two hours already". You can do it for an hour, then for an hour more. Maybe it's easier if you think that Russian prefixed verbs are similar to English phrasal verbs. To turn on(off) the lights is включить or выключить (literally in-key and out-key). "Вы-" is out. "Выучить" is out-learn. In English it would be "to learn out" but you dont' have such a verb. You have "to find out":-) For learn-1 instead of "finally I learned them well" we say "finally I learned them out well". "хорошо выучить" means "learn them well", it means all of this in your memory, you know them well now. "хорошо учить" is less idiomatic, it means, you're efficient in your learning. "учить почти час" means "for almost an hour". This "for an hour" can not be combined with learn-1 at all!!! *выучить почти час could only mean "to learn almost an hour of them", when you're learning by heart contents of a hour-long tape.

Райн хорошо выучит новые слова, потому что он учил их почти час. Райн хорошо выучил новые слова, потому что он учил их почти час. Both of these are correct. Your choice of the aspect depends on the context