Year 1 kids homework question. Do you know the answer, page-3

Silver. It is also an adjective. (although I'm guessing most 1st graders don't know what an adjective is). Also not directly quantifiable, can have 1 or 2 friends, 8 desks, 2 toothbrushes and 12 eggs etc. but you can't have 3 silvers.

Toothbrush. It is the only compound word. Again, maybe a concept a bit advanced for 1st graders.

Egg. It is the only word with a 'double' letter. Also the only food. That's a level 1st graders can comprehend. (Unless, of course, you're into cannibalism, in which case a friend could also be food!)

As for Friend, that is still a 'concrete' noun. The abstract noun form is friendship. Could be the only noun that can change it's state though. Tomorrow Jimmy may not be my friend, but my desk will still be a desk.

Could simply be an exercise in reason, no right or wrong answers, but gets you thinking about attributes associated with words.