Is it always possible to identify something as an element, compound, pure substance or mixture just by looking at it? explain using examples from the tables. (table attached)

What is conserved in the reaction shown below? h2(g) + cl2 (g) --> 2hcl(g)a. mass onlyb. mass and moles onlyc. mass, moles, and molecules onlyd. mass, moles, molecules, and volume