The question asks about the number of structural isomers of pentane, which is a hydrocarbon with five carbon atoms. Structural isomers are compounds that have the same molecular formula but differ in their arrangement of atoms.

Let`s examine each option to determine the correct answer:

Option 1: 5 - This option suggests that there are five structural isomers of pentane. However, this is incorrect because there are actually fewer isomers.

Option 2: 4 - This option states that there are four structural isomers of pentane. This is the correct answer. The four isomers of pentane are n-pentane, isopentane, neopentane, and two methylbutanes (2-methylpentane and 3-methylpentane).

Option 3: 2 - This option implies that there are only two structural isomers of pentane. This is incorrect because there are more than two isomers.

Option 4: 3 - This option suggests that there are three structural isomers of pentane. This is not accurate because there are actually four isomers, as mentioned earlier.

In conclusion, the correct answer is option 2, which states that there are four structural isomers of pentane.