Which term describes the action of obstructing an entrance or opening?

The term that best describes the action of obstructing an entrance or opening is "block." When you block an entrance, you obstruct access in a way that prevents passage, which aligns with the definition of the term. It highlights the concept of creating an impediment or barrier in that space.

"Close" typically refers to fully shutting something, such as a door or window, which may not necessarily imply a barrier as it could also mean placing something in its proper position or arrangement rather than obstructing it. "Seal" generally implies making something airtight or watertight, and while it can suggest preventing access, it doesn't directly convey the idea of obstructing an opening in the same way "block" does. "Bar" suggests a stronger or more permanent barrier, like a physical barrier or a restriction, but "block" is more commonly understood in everyday language to denote the action of obstructing access.