Please line up the round plates on the desk.
The room with the round window is the princess's.
I’m afraid of the full moon.
This word is made up of a single kanji with hiragana attached to it. That hiragana ends with い which means it's an adjective. So, what's the adjective form of the kanji that's in this word? 円 means both "yen" and "round" but in this case we're looking at the round one (because something isn't going to be "yenny"). The adjective version of round is round or circular. Just like a yen coin, this "round" is a flat round. As in, if you draw a circle on a piece of paper, you could use this word. Same goes with yen, which is why "yen" and "round" both use the same kanji.
Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You didn't learn that reading with this kanji, so here's a mnemonic to help you:
The reading for the 円 portion of this word is the same as a kanji you're learning in this level: 丸. That kanji means something really similar to this kanji. And guess what? They also share a reading. So, if you know that the reading of 丸 is まる then you'll also know that the reading for this word is まる!