Which of the following compounds is an Alicyclic compound?

The correct option is Cyclopropane.

An alicyclic compound is an organic compound that is both aliphatic and cyclic. This means it contains one or more carbon rings, which can be saturated or unsaturated, but do not have aromatic character (like benzene).
Let us analyze each of the given options:

1. Cyclopropane: It consists of three carbon atoms joined together in a ring form to shape a triangle, with the chemical formula $C_3{H}_6$. Since it contains a ring of carbon atoms and behaves like an aliphatic compound, it is classified as an alicyclic compound.

2. Isobutane: It is a branched-chain alkane with the chemical formula $\left({\mathrm{CH}}_3\right){\mathrm{CH}}_3$. It is an open-chain (acyclic) aliphatic compound, not cyclic.

3. Acetic acid: It is a simple carboxylic acid with the chemical formula ${\mathrm{CH}}_3\mathrm{COOH}$. It is an open-chain (acyclic) compound, not cyclic.

4. Ethane: It is a simple straight-chain alkane with the chemical formula $C_2{H}_6$. It is also an open-chain (acyclic) aliphatic compound.

Therefore, among the given options, only Cyclopropane is an alicyclic compound.