Panama is a good example of a long, narrow strip of land that joins two land masses surrounded by sea on either side. What is this kind of geographical feature called?

Isthmus is the correct answer. An isthmus is a narrow strip of land connecting two larger land areas and bordered by water on two sides. Panama is indeed a classic example of an isthmus, connecting North and South America. An estuary is where a river meets the sea, an island is surrounded by water, and a knoll is a small hill. The question correctly describes an isthmus.