"I was tired because I climbed a mountain."
but, but, but....
you can say this with the "because," like 山にのぼってからつかれました, explicitly saying that climbing the mountain was the cause. So does the sentence really seem wrong if there's a から to make the connection? Does the sentence really have the same meaning if you leave the から out? Something about that feels very imprecise.
I don't see a strong causative statement in the answer. This question came on the heels of numerous "から" exercises, so I was surprised that "kara" wasn't among the kana choices. However, "んです(ndesu)" was in the choices offered, so I answered "山にのぼってつかるましたんです" but it wasn't accepted. I don't understand why not?