DNA synthesis can be specifically measured by estimating the incorporation of radio labelled thymidine.

Introduction:
DNA synthesis refers to the process of replicating DNA molecules in living organisms. It is a crucial step in cell division, growth, repair, and reproduction. To measure DNA synthesis, researchers often use radioactively labelled nucleotides that can be incorporated into the newly synthesized DNA strands. By measuring the incorporation of these radio-labelled nucleotides, scientists can quantitatively assess the extent of DNA synthesis.

Explanation:
Among the given options, estimating the incorporation of radio-labelled thymidine is the most specific and reliable method to measure DNA synthesis. Here's why:

1. Radio-labelled nucleotides:
Radio-labelled nucleotides are used to track the incorporation of nucleotides into newly synthesized DNA strands. These nucleotides contain a radioactive isotope, such as tritium (^3H) or carbon-14 (^14C), which emits radiation that can be detected and measured.

2. Thymidine as a precursor:
Thymidine is one of the four nucleotides that make up DNA, along with adenine, guanine, and cytosine. During DNA synthesis, thymidine is incorporated into the growing DNA strand by DNA polymerase enzymes. By using radio-labelled thymidine, researchers can specifically measure the incorporation of this nucleotide into DNA molecules.

3. Specificity:
While other nucleotides like uracil and adenine are also involved in DNA synthesis, their incorporation can occur in other cellular processes as well. For example, uracil is not normally present in DNA but can be incorporated into RNA during transcription. Adenine is present in both DNA and RNA. Therefore, measuring the incorporation of these nucleotides may not provide a specific measurement of DNA synthesis.

4. Reliability:
Thymidine, being a specific precursor of DNA, is primarily incorporated into DNA during replication and repair processes. Therefore, estimating the incorporation of radio-labelled thymidine provides a reliable measurement of DNA synthesis.

Conclusion:
In conclusion, DNA synthesis can be specifically measured by estimating the incorporation of radio-labelled thymidine. This method provides a reliable and specific measurement of DNA synthesis as thymidine is a specific precursor of DNA and is primarily incorporated into DNA during replication and repair processes.