Here's why:

* Pentanetriol refers to a five-carbon chain with three hydroxyl (-OH) groups.

* Isomers are molecules with the same molecular formula but different arrangements of atoms.

Let's analyze the possible arrangements:

1. 1,2,3-Pentanetriol: OH groups on carbons 1, 2, and 3.

2. 1,2,4-Pentanetriol: OH groups on carbons 1, 2, and 4.

3. 1,3,5-Pentanetriol: OH groups on carbons 1, 3, and 5.

4. 2,3,4-Pentanetriol: OH groups on carbons 2, 3, and 4.

Notice that the positions of the OH groups are what differentiate the isomers. There are no other possible arrangements that would be unique molecules with the same molecular formula.