What number should be substituted for the question mark to balance the equation and why is that the correct number?

c3h8 +? o2 → 3co2 + 4h2o + heat

a.
the number 7 is needed to balance the equation. the right side shows 3 oxygen atoms in carbon dioxide and 4 oxygen atoms in water for a total of 7 oxygen atoms.

b.
the number 3 is needed to balance the equation. the right side of the equation shows 6 oxygen atoms, so the left side needs 3 o2.

c.
the number 5 is needed to balance the equation. the right side of the equation shows 10 oxygen atoms, so the left side needs 5 o2.

d.
the number 4 is needed to balance the equation. the right side shows 4 oxygen atoms, so the left side needs 4 oxygen atoms.