**Core Concept**
Sampling methods can be categorized based on the way the sample is selected from the population. The question describes a scenario where every fourth student is selected, which is a form of systematic sampling.

**Why the Correct Answer is Right**
Systematic sampling involves selecting samples based on a fixed interval or system. In this case, every fourth student is selected, which ensures that the sample is representative of the population. This method is useful when the population is large and the data is collected at regular intervals. The fixed interval helps to reduce bias and ensures that the sample is evenly distributed.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the description provided in the question. There is no mention of random sampling or probability sampling, which are the other two main categories of sampling methods.
**Option B:** Stratified sampling involves dividing the population into subgroups and then sampling from each subgroup. This method is used to ensure that the sample is representative of the entire population, but it does not involve selecting samples based on a fixed interval.
**Option C:** Cluster sampling involves selecting a group of individuals or items (a cluster) and then sampling from that cluster. This method is used when the population is difficult to access or when the data is collected at a specific location. However, it does not involve selecting samples based on a fixed interval.

**Clinical Pearl / High-Yield Fact**
Systematic sampling is a useful method for reducing bias and ensuring that the sample is representative of the population. However, it requires careful consideration of the sampling interval to ensure that the sample is evenly distributed.

**Correct Answer:** D.