5. Which of the following represents a compound H H-3 H2O O-16?

HH3 would be a Hydrogen-1 (protium) atom bound to a Hydrogen-3 (tritium) atom - since both are just isotopes of hydrogen it doesn't qualify as a compound.

By the same token, the oxygen would not be a compound

H2O is water - with 2 different elements it qualifies as a compound.