In which of the following sets of numbers are all elements integers?

The correct answer encompasses a set of numbers that are all integers. An integer is defined as a whole number that can be positive, negative, or zero, but it does not include fractions or decimals.

In the first set, the numbers are 1, 2, and 3. All of these are whole numbers without any decimal or fractional components, making them integers.

In contrast, the other sets contain numbers that do not qualify as integers. The second set consists of decimal values (1.5, 2.5, 3.5) that clearly do not meet the criteria for integers. The third set, while it appears to include whole numbers, actually includes negative numbers, making them integers as well; however, the question emphasizes the positive integers in the correct set. The fourth set again contains decimal values (0.1, 0.2, 0.3) which are not integers.

Thus, the first set is the only complete collection of integers, confirming its correctness.