Which of the following would increase the density of an object?

I don’t see any followings but two that would increase the density would be Increase mass and decrease volume.

hydrogen is a compound or a single element and a element is a pure substence that cannot be separated into simpler subtances

clorine is a compound its a chemical substance that forms when two or more different elements combine

so both hydrogen and clorine are compounds

c. establishing proper sleep hygiene