The actinide contraction is due to:
(A) Perfect shielding of $5f$ electrons.
(B) Imperfect shielding of $4f$ electrons.
(C) Imperfect shielding of $5f$ electrons.
(D) Perfect shielding of $4f$ electrons.

Hint – Start by describing actinides, include some special traits that actinides show. You can also add some general use of actinides. Then describe what actinide contraction is and define the two reasons leading to it.

Complete step by step solution:
> Actinides are the parts of the periodic table that contain elements with atomic numbers from 90 to 103. All actinides are radioactive in nature. Actinides include a wide range of chemical elements like protactinium, thorium, transuranic and thorium. Actinides also contain chemical species produced by nuclear reactions. The name of actinides comes from the name of the first element in the actinide series “actinium”.
> These actinide elements are used in energy production in nuclear power plants. This energy production also leads to a major nuclear waste problem and that is harmful for the environment. Actinides are also used in nuclear weapons and also safety equipment like modern smoke detectors.
> Actinide Contraction - Actinides like lanthanides show a certain tendency to contract in their atomic radii, this is called actinide contraction. The atomic size and ionic radii of tri positive actinides decrease regularly as we move along the actinide series. As we move from $Th$ to $Lw$ the atomic/ionic radii steadily decreased due to the following reasons.
> The nuclear charge steadily increases and so does the force of attraction between the electrons and the nucleus (the protons). This leads to a shrink in the size of radii of the orbits in which the electrons revolve and hence the size of atomic/ionic radii decreases.
In case of actinides electrons can move in the inner (n-2) f orbital, due to the imperfect shielding of the $5f$ shell’s electron. So if the electron moves on the inner side the size of atomic/ionic radii decreases accordingly.

Note - Actinide elements being radioactive in nature, release a considerably large amount of energy when they decay radioactively. Uranium and thorium are found in abundance on earth actinide, whereas chemical species like plutonium can only be obtained synthetically prepared in a carefully controlled environment.