endless is correct because a Penelope's web means a task or work that seems endless because it is repeatedly undone or never finished.

This idiom or phrase is useful because it helps students understand the intended meaning beyond the individual words.

Examples: The repairs became a Penelope's web because each fix created another problem. For some students, rewriting the report felt like a Penelope's web.

The idiom emphasizes never-ending work, not simply difficulty or decline.