The correct option is diurnal.

To understand why this is the correct antonym, let's look at the definitions of the words:
1. Nocturnal: Relating to, occurring, or active in the night (e.g., nocturnal animals like owls are active at night).
2. Diurnal: Relating to, occurring, or active during the daytime (e.g., diurnal animals are active during the day). Since diurnal represents the exact opposite behavior and time period of nocturnal, it is its direct antonym.

Let's also define the other options to see why they are incorrect:
- Normal: Conforming to a standard, usual, typical, or expected state or condition.
- Abnormal: Deviating from what is normal or usual, typically in a way that is undesirable or worrying.
- Exceptional: Unusual; not typical; out of the ordinary or outstandingly good.

Therefore, the direct antonym of nocturnal is diurnal.