**Core Concept**
Ascorbic acid, also known as vitamin C, is an essential nutrient that humans and other primates, as well as guinea pigs, cannot synthesize due to the absence of the enzyme L-gulonolactone oxidase. This enzyme is necessary for the conversion of glucose to ascorbic acid in the body.

**Why the Correct Answer is Right**
Ascorbic acid plays a crucial role in various bodily functions, including collagen synthesis, iron absorption, and the proper functioning of the immune system. The absence of L-gulonolactone oxidase in humans and other primates, as well as guinea pigs, renders them dependent on dietary sources of vitamin C to meet their nutritional requirements. This is in contrast to other animals, such as most mammals and birds, which possess this enzyme and can synthesize their own vitamin C.

**Why Each Wrong Option is Incorrect**
**Option A:** Pantothenic acid, also known as vitamin B5, is synthesized in the human body and is not dependent on dietary sources. It plays a crucial role in the synthesis of coenzyme A and is essential for various metabolic processes.
**Option B:** Nicotinic acid, also known as niacin or vitamin B3, is synthesized in the human body from the amino acid tryptophan and is not dependent on dietary sources. It plays a crucial role in the synthesis of NAD and NADP, which are essential for various metabolic processes.
**Option D:** Folic acid, also known as vitamin B9, is synthesized in the human body from the amino acid glycine and is not dependent on dietary sources. It plays a crucial role in DNA synthesis and repair.

**Clinical Pearl / High-Yield Fact**
It's essential to note that humans and other primates, as well as guinea pigs, are unable to synthesize ascorbic acid due to the absence of L-gulonolactone oxidase. This makes them susceptible to scurvy, a disease caused by a deficiency in vitamin C, which can lead to symptoms such as fatigue, malaise, and connective tissue weakness.

**✓ Correct Answer: C. Ascorbic acid**