## Core Concept
The question pertains to the mode of transmission of diseases from mother to child through vectors. This concept falls under the realm of **transovarial transmission**, a specific type of disease transmission.

## Why the Correct Answer is Right
**Transovarial transmission** refers to the passage of pathogens, such as bacteria or viruses, from a female vector (e.g., mosquito) to her offspring through eggs. This is a critical concept in understanding the life cycle of certain diseases and their vectors. The correct term for mother to child transmission in vectors is indeed **transovarial transmission**.

## Why Each Wrong Option is Incorrect
* **Option A:** While some diseases are transmitted through direct contact or vectors, the term is not specified for mother to child transmission through vectors.
* **Option B:** This option does not directly relate to the transmission through vectors.
* **Option C:** This could potentially relate to vertical transmission, which is a broader term for mother to child transmission but not specifically through vectors.

## Clinical Pearl / High-Yield Fact
A key point to remember is that **transovarial transmission** is crucial for the maintenance of certain diseases in nature, such as **tick-borne** and **mosquito-borne diseases**. For example, the **Lyme disease** bacteria can be transmitted transovarially in ticks. Understanding this concept can help in developing strategies for disease control.

## Correct Answer Line
**Correct Answer: D. Transovarial transmission.**