Final answer:
The primary symptom of niacin toxicity is flushing, a reddening of the skin due to vasodilation. Other possible symptoms can include dizziness and gastrointestinal issues such as diarrhea.
The correct option is 4.
Step-by-step explanation:
The primary symptom of niacin toxicity is flushing. This is due to the vasodilatory effects of niacin, which can cause the blood vessels to dilate and result in a noticeable redness of the skin.
Other symptoms of niacin toxicity might include dizziness, gastrointestinal problems such as diarrhea, and other symptoms like peptic ulcer, hyperglycemia, and gout. However, flushing is often the most immediate and noticeable symptom when someone has taken an excessive amount of niacin.
The correct option is 4.