Actinides [1994]
a) Are all synthetic elements
b) Include element 104
c) Have any short-lived isotopes
d) Have variable valency

The correct answer is option 'D', which states that actinides have variable valency.

Explanation:

1. Actinides are all synthetic elements:
- Actinides are a series of elements in the periodic table that follow the element actinium (Ac) and precede the element lawrencium (Lr). These elements are all synthetic, which means they do not occur naturally on Earth and are instead created through nuclear reactions in laboratories.

2. Actinides include element 104:
- Element 104, also known as Rutherfordium (Rf), is indeed one of the actinides. It was first synthesized in 1964 and is highly unstable, with a half-life of only a few seconds. Rutherfordium is named after the physicist Ernest Rutherford.

3. Actinides have short-lived isotopes:
- Isotopes are different forms of an element that have the same number of protons but different numbers of neutrons. Actinides, being synthetic elements, often have isotopes with short half-lives. A half-life is the time it takes for half of the atoms in a radioactive substance to decay. Many actinides have isotopes with half-lives ranging from seconds to days.

4. Actinides have variable valency:
- Valency refers to the number of electrons an atom can gain, lose, or share in order to form chemical bonds. Actinides have variable valency, meaning that their atoms can have different valence states depending on the specific chemical reaction or compound they are involved in. This variability in valency is due to the presence of f-orbitals in the electron configuration of actinide elements, which allows for the accommodation of a varying number of electrons.

In conclusion, the correct answer is option 'D' because actinides have variable valency. This characteristic sets them apart from other elements in the periodic table and contributes to their unique chemical behavior.