Question

Identify substance A.

Answer 1

I'd say that the image represents a cell.

Since you are asking for a substance, I suppose that the A is representing the cytoplasm, a substance that fills the cells interior. I assume it's the cytoplasm mainly because it's a soft jelly like material, that matches with the meaning of "substance".

Hope it helped,

BioTeacher101

(If you have any questions, feel free to ask them in the comments below)

Answer 2

I feel like we are missing some context here for the picture, but I will do my best:

I am assuming here that it's a cell.

It's a eukartyotic cell - it has a clear nucleus, if it didn't; it would be a prokaryotic cell.

Some elements in the eukaryotic cells are ribosomes and mitochondria, but mitochondria are bigger than ribosomes, so it's more likely that the A is mitochondria - and that the scale is too small to show the ribosomes.