Which of the following is not a swimming stroke ?

The swimming stroke that is not listed among the options is the downstroke.

Let's briefly discuss the other three swimming strokes to understand why the downstroke is not considered a swimming stroke:

Crawl: Also known as freestyle, the crawl is the fastest and most commonly used stroke in competitive swimming. It involves a continuous alternating arm motion and a flutter kick. Swimmers move their arms in a windmill-like pattern, pulling the water towards them, while their legs kick in a repetitive up and down motion.

Backstroke: As the name suggests, swimmers swim on their backs, facing up. The main propulsive force in backstroke comes from the alternating arm movement, similar to the crawl stroke. The arms move in a wider arc, and the hands enter the water pinky-first. The legs perform a flutter kick, similar to that of the crawl stroke.

Breaststroke: The breaststroke is the slowest of the four competitive swimming strokes, but it is often considered the most graceful. Swimmers synchronize a frog-like kick and an arm pull to propel themselves through the water. The arms move together in a half-circle motion underwater, while the legs kick outwards and then back inwards simultaneously.

Downstroke: The term "downstroke" does not correspond to any recognized swimming stroke. It may be a term used in other contexts or in certain swimming drills, but it is not a distinct swimming stroke like the crawl, backstroke, or breaststroke.