What term describes a headland or promontory of large size extending into a body of water?

The term that describes a headland or promontory of large size extending into a body of water is "cape." A cape is typically characterized by its significant geological prominence and is often a notable landmark for navigation. Capes can be formed by the erosion of the land by waves, currents, and tides, leading to their distinguishing shape that protrudes into an ocean, sea, or other large bodies of water.

The other options refer to different geographical features: a bay is a body of water partially enclosed by land, an island is a landmass surrounded by water, and a coast refers to the land along the edge of a body of water. Each of these terms describes distinct phenomena, highlighting the specificity of the term "cape" to describe a substantial landform that extends into water.