Which of the chemicals in the reaction is classified as a diatomic molecule? Why

A.Water
G. Hydrogen
C. Methanol
D. Carbon dioxide

Methanol (CH 3 OH) can be produced by the following reaction:
CO 2 + 3H 3 ---> CH 3 OH + H 2 O

Hydrogen = H2 is diatomic

Not sure what you are asking in the second bit, but
"CO 2 + 3H 3 ---> CH 3 OH + H 2 O" is messed up.
Try
CO2 +3 H2 ---> CH3OH + H2O
TO get that [ie, balance the equation] start with
CO2 + x H2 ---> y CH3OH + z H2O
You can see [by conserving Carbon atoms] that y = 1
Then by conserving oxygen atoms, you can see z = 1
Then total the Hydrogens on the right side and you can see x = 3

OK. The only diatomic molecule is hydrogen because single atoms of it are covalently bonded to each other and appears in nature as H2.
The others have 3 or more atoms making up the molecule.

Not sure what your point is on the Methanol question other than maybe balancing it.

CO2 + 3H2 ---> CH3OH + H2O

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