D) Zirconium is not a radioactive element.

Explanation:
Radioactive elements are those elements that undergo spontaneous radioactive decay, emitting radiation in the process. Zirconium, on the other hand, is not a radioactive element. Let's take a closer look at the other options to understand why they are radioactive.

a) Uranium:
Uranium is a radioactive element with atomic number 92. It is well-known for its radioactivity and is commonly used as a fuel in nuclear reactors. Uranium undergoes radioactive decay, producing various daughter isotopes in the process.

b) Radium:
Radium is also a radioactive element. It has atomic number 88 and is highly radioactive. Radium was once used in medical treatments and luminescent paints, but its use has significantly decreased due to its high toxicity.

c) Thorium:
Thorium is another radioactive element, with atomic number 90. It is commonly found in monazite sands and has a long half-life, making it suitable for use in nuclear reactors as a fuel. Thorium is considered a potential alternative to uranium for nuclear power generation.

d) Zirconium:
Zirconium, unlike the other options, is not a radioactive element. It has atomic number 40 and is known for its corrosion resistance and low neutron absorption cross-section, making it useful in nuclear applications. Zirconium is commonly used in the construction of nuclear reactors and as a cladding material for fuel rods.

e) None of the above/More than one of the above:
This option is incorrect since only Zirconium is not a radioactive element among the given options. Uranium, Radium, and Thorium are all radioactive elements.

In conclusion, Zirconium is not a radioactive element, while Uranium, Radium, and Thorium are all radioactive elements.