Answer: The correct answer is Option a.
Step-by-step explanation:
An equinox is defined as an event that occurs when Sun's rays fall on the equator at noon and at an angle of 90°.
The axis of rotation of Earth must be perpendicular to the Sun's rays for this event to occur.
If the axis is titled to some angle even, then equinox will not occur.
Hence, the correct answer is Option a.