There are five isomers of C7H8O that have a phenyl ring. To understand this, let's first analyze the molecular formula C7H8O.

Molecular Formula Analysis:
- C7H8O indicates that the molecule contains 7 carbon (C) atoms, 8 hydrogen (H) atoms, and 1 oxygen (O) atom.
- It does not provide any specific information about the arrangement or connectivity of these atoms.

Identifying the Phenyl Ring:
- The phenyl ring, also known as a benzene ring, consists of six carbon atoms bonded in a cyclic structure with alternating single and double bonds.
- By analyzing the molecular formula C7H8O, we can determine that one of the carbon atoms must be part of the phenyl ring since there are only 7 carbon atoms in total.
- This leaves 6 carbon atoms remaining, which can form various arrangements with the other atoms present in the molecule.

Types of Isomers:
1. Positional Isomers:
- Positional isomers differ in the position of functional groups or substituents on the carbon chain.
- In this case, we are specifically looking for isomers with a phenyl ring, so the position of the ring will vary.

2. Skeletal Isomers:
- Skeletal isomers have the same molecular formula but different carbon skeletons or arrangements.
- In this case, the carbon skeleton will differ, as the phenyl ring can be connected to different carbon atoms in the chain.

3. Functional Group Isomers:
- Functional group isomers have the same molecular formula but different functional groups attached.
- Since we are only considering a phenyl ring, this type of isomer is not applicable.

Determining the Number of Isomers:
- To determine the number of isomers that have a phenyl ring, we need to consider the possible positions where the ring can be attached to the carbon chain.
- The phenyl ring can be attached to any of the 6 remaining carbon atoms, resulting in 6 possible isomers.
- However, one of these isomers is equivalent to another when the molecule is rotated, resulting in identical structures. Thus, we need to divide by 2 to account for this symmetry.
- Therefore, there are a total of 6/2 = 3 isomers with a phenyl ring.

Final Answer:
- There are three isomers of C7H8O that have a phenyl ring.

However, the correct answer mentioned is '5', which contradicts the above explanation. It is possible that there might be additional considerations or information not provided in the question that lead to the answer '5'.