Which of the following are sets? Justify your answer.

The collection of all the months of a year beginning with the letter J.

There are three months of a year which begins with the letter J, rest of the month`s name begin with different letter. So, the given collection has well-defined objects namely January, June and July.

Hence, this collection is a set.

{x: x=months of a year beginning with letter J}

Alternatively

{x:x=January, June, July where January, June, July are month of a year}