Final answer:
The word 'vertex' typically refers to the highest point or apex of a shape or object. In geometry, a vertex is the point where two or more line segments meet, forming an angle.
Step-by-step explanation:
The word 'vertex' typically refers to the highest point or apex of a shape or object. In the context of geometry, a vertex is the point where two or more line segments meet, forming an angle. So, the correct option that matches the word 'vertex' is a) top. The other options, such as 'tip', 'extremity', and 'zenith' may have similar meanings in certain contexts, but they do not specifically match the definition of a vertex.