The correct answer is option 1: Ascorbic acid. Ascorbic acid, also known as vitamin C, is essential for maintaining healthy gums and preventing bleeding. It plays a crucial role in collagen synthesis, which is necessary for the formation of healthy connective tissues in the gums. Ascorbic acid helps strengthen the blood vessels in the gums, making them less prone to bleeding.

Option 2, Vitamin B, is not directly involved in preventing bleeding gums. However, certain B vitamins, such as vitamin B12, can help maintain overall oral health and reduce the risk of gum disease, which can cause bleeding gums.

Option 3, Nicotinamide, is a form of vitamin B3 that is involved in various cellular processes but is not specifically known to prevent bleeding gums.

Option 4, Thiamine, also known as vitamin B1, is vital for overall health but does not have a direct role in preventing bleeding gums.