**Core Concept**
DNA cloning involves the use of vectors to replicate and express specific genes of interest. Expression vectors are designed to drive the expression of the cloned gene, whereas cloning vectors are primarily used for the replication and amplification of the DNA insert.

**Why the Correct Answer is Right**
A selection marker, such as antibiotic resistance or a reporter gene, is necessarily present in the expression vector to facilitate the identification and isolation of cells that have successfully taken up and expressed the cloned gene. This is particularly important in the context of expression vectors, where the goal is to drive the expression of the gene of interest. In contrast, cloning vectors may not necessarily contain a selection marker, as the primary goal is to amplify and replicate the DNA insert.

**Why Each Wrong Option is Incorrect**
**Option A:** A promoter is necessary for the initiation of transcription, but it is not unique to expression vectors.
**Option B:** A multiple cloning site is a common feature of both cloning and expression vectors, as it provides a convenient location for the insertion of the DNA fragment of interest.
**Option C:** A terminator is involved in the termination of transcription, but it is not a distinguishing feature of expression vectors.

**Clinical Pearl / High-Yield Fact**
When working with expression vectors, it's essential to ensure that the selection marker is compatible with the host cell line being used, as this will impact the efficiency of gene expression.

**Correct Answer: D. A selection marker.**