A long, winding ridge of stratified sand and gravel is known as:

The correct answer is esker.

Explanation:
An esker is a long, sinuous or winding ridge composed of stratified sand and gravel. Eskers are formed by subglacial or englacial meltwater streams flowing through tunnels within or beneath a melting glacier. As the flowing water carries sediment, it deposits sorted layers of sand and gravel inside the channel. When the surrounding ice melts completely, these stream-bed deposits are left behind as prominent, winding ridges elevated above the surrounding terrain.

Analysis of incorrect options:
• Moraine: A ridge or mass of unsorted glacial debris (till) deposited directly by a glacier without meltwater sorting.
• Erratic: A piece of rock or large boulder transported by ice that differs in rock type from the local bedrock where it was deposited.
• Aquifer: An underground layer of water-bearing permeable rock, sand, or gravel, which is a hydrogeological feature rather than a specific glacial landform ridge.