Which of the following pairs of words have a similar relationship?

The relationship between the words "Decrease" and "Increase" is one of opposites or antonyms. These words directly relate to the concept of change in quantity or size, where "decrease" refers to making something smaller or lesser, while "increase" refers to making something larger or greater.

In contrast, the other word pairs do not exhibit a similar relationship. "Stretch" and "Enlarge" are more synonymous, as they both refer to the action of making something bigger. "Sitting" and "Walking" represent two different forms of physical activity but do not oppose or relate to each other in the same way that an action and its opposite would. Lastly, "Square" and "Triangle" are geometric shapes that are distinct from each other but do not have a relational dynamic like increase and decrease do. Therefore, the best answer is indeed the pair of words that represent opposing concepts.