**Core Concept**
The question is testing the understanding of factors influencing dental caries activity. Dental caries is a bacterial infection that causes demineralization of tooth enamel, leading to cavities. The activity of caries refers to the rate at which the disease progresses.

**Why the Correct Answer is Right**
Caries activity is increased in the presence of high levels of fermentable carbohydrates in the diet. These carbohydrates are broken down by oral bacteria, such as Streptococcus mutans, into acids like lactic acid. The acid production increases the pH in the oral cavity, making it more conducive for the growth and activity of cariogenic bacteria. This process is mediated by the enzyme glucosyltransferase, which helps in the formation of dental plaque.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not provided.
**Option B:** This option is not provided.
**Option C:** This option is not provided.
**Option D:** This option is not provided.

**Clinical Pearl / High-Yield Fact**
The presence of fluoride in the water supply or toothpaste can significantly reduce the activity of caries by inhibiting the activity of glucosyltransferase and making teeth more resistant to acid attacks.

**Correct Answer:** A. A diet high in fermentable carbohydrates.