The rotting of fruits is (a desirable/an undesirable) change.
an undesirable
Explanation for correct option -
Option (b) Undesirable change is defined as a change that is damaging to humans or causes destruction. So, the rotting of fruits is not good for our health and thus it's an undesirable change.
Explanation for incorrect option -
Option (a) is not the correct option as changes which are desired to occur (i.e. we want them to take place) are known as desirable changes. So, rotting of food is not desirable.
Therefore, option (b) is correct.