Which of the following sets contains only integers?

The set that contains only integers is characterized by having whole numbers that can be either positive, negative, or zero, but not fractions or decimals.

The first option includes -3, -2, -1, 0, and 1. All these numbers are whole numbers, with no decimal or fractional part, making them pure integers.

In contrast, the other options contain numbers that are not integers. For instance, the second set includes -3.4, which is a decimal, thus not an integer. The third set has 3.5, another decimal, disqualifying it as a set of integers. Finally, the fourth set features 2.8, which is also not an integer due to its decimal component.

The clarity of the first option as a complete set of integers is what makes it the correct choice.