Step-by-step explanation:
Structurally alcohols are alkanes with one H atom replaced by an -OH group. The main difference with diols is that they have two alcohol groups.
If we take for example the first row. The alcohol has 4 atoms of carbon, so its name is 1-butanol. In the same row, we are given a diol that only has 3 atoms of C (one less). Its name is 1,3-propanediol.
If we consider all the rows we will see that the main difference between the structures that we are given is that in each diol one atom of C is replaced by an alcohol group (-OH).
Answer: the main difference between the structures that we are given is that in each diol one atom of C is replaced by an alcohol group (-OH).