**Core Concept**
DNA labelling is a technique used to attach a detectable molecule to the DNA molecule, allowing for its visualization, quantification, or tracking in a variety of biological and experimental contexts. This technique is crucial in molecular biology, cell biology, and genetic research.

**Why the Correct Answer is Right**
The correct answer involves the use of nucleotide analogues, specifically **bromodeoxyuridine (BrdU)**, which is incorporated into the DNA during the S phase of the cell cycle. BrdU has a bromine atom substituted for the hydrogen atom at the 5-position of the pyrimidine ring, making it distinguishable from the naturally occurring thymidine. This allows researchers to specifically label and detect newly synthesized DNA.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because **thymidine** is a naturally occurring nucleotide and not a labelled version of DNA.
**Option B:** This option is incorrect because **adenosine triphosphate (ATP)** is a molecule involved in energy transfer and not a DNA labelling agent.
**Option C:** This option is incorrect because **deoxyribonuclease (DNase)** is an enzyme that breaks down DNA and not a DNA labelling agent.

**Clinical Pearl / High-Yield Fact**
BrdU labelling can be used to track the proliferation of cells, especially in cancer research, and to study the effects of various treatments on cell growth and survival.

**Correct Answer:** C.