The number of hydrogen atom(s) attached to phosphorus atom in hypophosphorous acid is

Hypophosphorous acid is represented by the molecular formula $$H_3PO_2$$.

To locate each hydrogen, first recall that phosphorus shows a coordination number of 4 in this acid (valence shell 3s$$^2$$3p$$^3$$ ⇒ total of 5 valence electrons; one is used for a P=O double bond, one for a P-O single bond, and the remaining two for P-H single bonds).

Thus the conventional (Lewis) structure is
P bonded to: one doubly bonded oxygen (P=O), one hydroxyl group (P-O-H), and two hydrogens directly attached to phosphorus (P-H).

Therefore, exactly two hydrogen atoms are directly attached to the phosphorus atom.

Option B which is: two