I have my kindergartners explain their thinking when we solve word problems. Here are some examples of things I often hear.
I used the number line. I started on 8 and then went backwards four because the problem said they started with 8 and then four went away.
I used my fingers. (I then ask how they used their fingers, and hope for an explanation!).
I used the tens frame. First I put 8 on the tens frame, and then I took four off because it said four went away.
And my very favorite - I used my brain.
My high kids can give a very detailed explanation of their thinking. My grade level kids can tell me what they did, but can't always tell me why without some prompting.
My low kids can't even solve the problem.
I think I'd have pretty high expectations that a first grader could orally explain their answer. Writing it is a whole different ballgame.