Which of the following is a radioactive element? 

Plutonium 

The correct answer is option 3. Plutonium.

Plutonium is a radioactive element, and it is represented by the symbol Pu on the periodic table. Here is a brief explanation:

1. Radioactivity:  Plutonium is known for its radioactive properties, meaning it undergoes spontaneous nuclear decay, emitting radiation in the process. This radioactivity is a result of the instability of certain isotopes of plutonium.

2. Isotopes:  Plutonium has several isotopes, but the most significant from a radioactivity standpoint is plutonium-239 (Pu-239). Pu-239 is fissile, meaning it can undergo nuclear fission, a process that releases a large amount of energy.

3. Nuclear Applications: Due to its radioactive nature and the ability to sustain a nuclear chain reaction, plutonium has been used in the development of nuclear weapons and as a fuel in nuclear reactors. It has both civilian and military applications.

In contrast, hafnium, rhodium, and sulfur are not inherently radioactive elements. Hafnium (Hf) and rhodium (Rh) are stable elements, and sulfur (S) has stable isotopes. It's important to note that the radioactivity of an element is determined by the specific isotopes it possesses, and not all isotopes of a given element may be radioactive.