Understanding Transition Metals' Melting Points
Transition metals are known for their variable melting points, influenced by their electronic structure and bonding. Among the options provided, the question targets the one with the lowest melting point.

Analysis of Options
- **Zn (Zinc)**:
- Melting Point: 419.5 °C
- Zinc is considered a transition metal but is often categorized as a post-transition metal due to its distinctive properties.
- **Cr (Chromium)**:
- Melting Point: 1907 °C
- Chromium has a high melting point due to strong metallic bonding and the presence of unpaired electrons.
- **Cd (Cadmium)**:
- Melting Point: 321.1 °C
- Cadmium, like zinc, is a post-transition metal and exhibits lower melting points compared to many transition metals.
- **Hg (Mercury)**:
- Melting Point: -38.83 °C
- Mercury is unique as it is a liquid at room temperature, indicating a very low melting point.

Conclusion
The correct answer to the question is **Hg (Mercury)**, which has a melting point lower than all the listed options. The confusion arises because while zinc is a transition metal, its melting point (419.5 °C) is not the lowest among the given choices. In contrast, mercury, with its -38.83 °C melting point, is indeed the transition metal with the lowest melting point.
In conclusion, the lowest melting point among the options provided is attributed to **Hg (Mercury)**, not Zn.