The compound butanol has the following structural formula.

A string of 4 C atoms are bonded above, left, and below to H. The right-hand end is bonded to O, which in turn is bonded to H.

Which of these is a structural isomer of butanol?
A string of 4 C atoms are bonded above, below, left and right to H.
A string of 4 C atoms is bonded above, below, left, and right to H, except the second C, which is bonded below to O, which is bonded below to H.
A string of 4 C atoms is bonded above, below, left, and right to H, but the chain is interrupted between the first and second C, which are bonded to an O between them.
A string of 4 C atoms is bonded above, below, and left to H, except the last C has no H below and is double-bonded to an O to the right.

A string of 4 C atoms is bonded above, below, left, and right to H, but the chain is interrupted between the first and second C, which are bonded to an O between them. OR A string of 4 C atoms is bonded above, below, left, and right to H, except the second C, which is bonded below to O, which is bonded below to H.

Explanation:

Butanol: 4C's, 10H's, 1 O

First option: 4C's, 10H's

Second option: 4Cs, 10H's, 1 O

Third option: 4C's, 10H's, 1 O

Fourth option: 4C's 8H's, 1 O

Structural isomers have the same molecular formula, or same number of atoms of each element.