Lets start with you first answer,
Hydrogen and phosphorus indeed does create a covalent compound of PH3 but the key here is that the bond has to be polar covalent. Looking at the dipole moment of the P-H bond, P has a electronegativity of 2.19 and hydrogen has a 2.2. Hence, there is barely any electronegativity difference. Barely enough to consider significant hence, it is wrong.
Second answer, potassium and sulfur.
They form a compound of sodium sulfide Na2S
However, this is an ionic compound as it consists of a metal and non-metal bonded together.
Hence, this is wrong.
Third answer, hydrogen and iodine
This forms a compound of hydrogen iodide, HI
This is a covalent compound as there is a bond formed via sharing of electrons between 2 non metal atoms.
Since iodine is significantly more electronegative that hydrogen, HI is indeed a polar covalent molecule. So this answer is correct.
Final answer oxygen and oxygen
This forms a diatomic molecule of oxygen gas.
Since both atoms are from the same element, they are of same electronegativity. Hence the bond formed between them although is covalent, is not polar.
Hence, this answer is wrong.