Which of the following exercises is characterized as vigorous?

Shoveling snow is characterized as a vigorous exercise because it requires significant effort, engages multiple muscle groups, and significantly raises the heart rate. This activity often involves quick, forceful movements that can lead to substantial energy expenditure, making it a demanding workout that can contribute to cardiovascular conditioning and calorie burning.

In contrast, walking, while beneficial for seniors' overall health, is generally considered a moderate-intensity exercise, especially at a leisurely pace. Swimming can vary in intensity depending on the stroke and pace used; it can range from moderate to vigorous but isn't classified as rigorously vigorous in all contexts. Light stretching, on the other hand, is predominantly a low-intensity exercise focused on improving flexibility rather than cardiovascular or muscular strength.

Thus, the hallmark of vigorous exercise is the level of exertion and heart rate increase, which is most prominent in shoveling snow among the options given.