A strait is a narrow water body that separates two land masses. Option 1 accurately defines a strait. A strait is usually formed by the movement of tectonic plates or erosion caused by water flow. It is distinct from an isthmus, which is a narrow strip of land separating two water bodies. Option 2 incorrectly refers to an isthmus rather than a strait. Option 3 describes a deep penetration of a water body into the land, which is not the definition of a strait. Therefore, option 1 is the correct answer. A well-known example of a strait is the Strait of Gibraltar, which separates Europe and Africa.