Question

which contains the lowest % of hydrogen?

HCl

H3PO4

H2SO4

H20

is it HCL?

Answer 1

I don't know. I think you must calculate %H in each to know. For HCl, it is
(atomic mass H/molar mass HCl)*100 = ?
For H3PO4, it is
(3*atomic mass H/molar mass H3PO4)*100 = ?
Etc.