Which of these is an Integer?

An integer is a whole number that can be positive, negative, or zero, and it does not include fractions or decimal points. In this case, the answer is -1, which fits the definition of an integer perfectly. Negative numbers are part of the integer set, so -1 qualifies as an integer.

The other options do not meet the criteria for integers. The number 3.14 is a decimal number and thus not an integer. The single quote around '3' indicates that it is a string representation of a number, which also disqualifies it from being an integer. Similarly, "-1" is enclosed in double quotes, making it a string rather than a numerical value, meaning it's not an integer either. Thus, -1 stands out as the only true integer in the list.