## Core Concept
Oncogenes are genes that have the potential to cause cancer. They can become activated (or "turned on") as a result of mutations. Studying oncogenes involves understanding their role in cell growth, division, and cancer development.

## Why the Correct Answer is Right
The correct answer involves understanding the methodologies used to study oncogenes. Oncogenes can be best studied by **transfection assays**. This technique involves transferring genetic material from one cell into another to study the effects of specific genes, such as oncogenes, on cell behavior. Transfection assays allow researchers to introduce an oncogene into a normal cell and observe if it leads to cancerous changes, thereby directly implicating the oncogene in cancer development.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because while certain techniques like PCR (Polymerase Chain Reaction) can be used to amplify specific DNA sequences, including oncogenes, for further study, they do not directly assess the oncogene's ability to transform cells.
- **Option B:** This option is incorrect as it does not provide a clear or standard method for studying oncogenes. Without specifics, it's hard to assess its relevance, but generally, studying oncogenes involves functional assays rather than just observation or non-specific "study."
- **Option C:** This option might seem plausible as sequencing can identify mutations in oncogenes. However, sequencing alone, while crucial for identifying potential oncogenic mutations, does not directly demonstrate the oncogene's role in cancer.

## Clinical Pearl / High-Yield Fact
A key point to remember is that **oncogenes act as dominant genes**, meaning that a single copy of the mutated gene can lead to cancer. This is in contrast to tumor suppressor genes, which act in a recessive manner, requiring both copies to be mutated or inactivated to lose their protective effects.

## Correct Answer Line
**Correct Answer: D. Transfection assays.**