(c) zero
By the right-hand thumb rule, the magnetic field due to the upper wire at P is directed out of the page, and the field due to the lower wire at P is directed into the page. Since both currents are equal and P is equidistant, the two fields are equal in magnitude but opposite in direction, giving a resultant of zero.
For two parallel wires carrying current in the same direction, apply the right-hand thumb rule to each wire separately at point P (midway between them). The field from the upper wire curves downward at P (into the page on one side), while the field from the lower wire curves upward at P (out of the page on the same side) — they cancel exactly. This is why the answer is (c) zero. Had the currents been in opposite directions, the fields would add up instead of cancelling.