Option 1 -> Mysterious refers to something unknown or puzzling, unrelated to the difficulty level of a task.
Option 2 -> Continuous means ongoing or uninterrupted, which doesn't capture the challenging nature implied by arduous.
Option 3 -> Supportive means providing help or encouragement, which is opposite to the demanding nature of arduous tasks.
Option 4 -> Difficult means requiring considerable effort and hard work, which directly matches the meaning of arduous.
Hence, Option 4: Difficult -> "Arduous" describes tasks or activities that are strenuous, exhausting, and require significant effort to complete, making "difficult" the closest synonym -> correct