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An isomer refers to a compound with the same molecular formula as another compound but with a different structural arrangement of the atoms. In other words, isomers are molecules that have the same number of atoms of each element, but the atoms are arranged in a distinct way, leading to different physical and chemical properties.

Let's analyze each of the given alkanes to check if they are capable of existing in isomeric forms. 1. Ethane: \(C_2H_6\) - Ethane has two carbon atoms and a single possible structural arrangement, thus it has no isomeric forms. 2. Propane: \(C_3H_8\) - Propane has three carbon atoms and, like ethane, has only one possible structural arrangement. So, it also has no isomeric forms. 3. Butane: \(C_4H_{10}\) - Butane has four carbon atoms, and there are two possible structural arrangements for this molecule: n-butane (normal butane), where all carbon atoms are in a straight chain, and iso-butane (2-methylpropane), where three carbon atoms are in a chain, and the fourth is connected to the second carbon in the chain. As there are two possible arrangements, butane has isomeric forms. 4. Pentane: \(C_5H_{12}\) - Pentane has five carbon atoms, and there are three possible structural arrangements: n-pentane (normal pentane), isopentane (2-methylbutane), and neopentane (2,2-dimethylpropane). As there are three possible arrangements, pentane also has isomeric forms. Among the four given alkanes, both butane and pentane are capable of existing in isomeric forms.