A small 50ml pill container.
This cache is part of the Blockbuster A to Z series.
An old 80's gameshow in which contestants had to pick letters and answer questions relating to that letter to get across the board and win.
If we were doing a modern day geocache version of this I would imagine it would go:
P&Vee: "I'll have an E please Natalie247."
Natalie247: "Which E is a type of cache at a special place that people can visit to learn about a unique geoscience feature of our planet?"
P&Vee: "It's E....."
Natalie247: "Correct - you might want to store that away for later to see where i go with it."