Explanation:

- Herculean task: This idiom refers to a task that is extremely difficult or requires great effort, much like the labors of Hercules from Greek mythology.

- Option 1: Very easy task: This option is incorrect as it contrasts with the true meaning of the idiom.

- Option 2: Very tough task: This option is correct. A Herculean task is indeed a very challenging or demanding task.

- Option 3: Very old task: This option does not align with the idiom's meaning, which refers to difficulty, not age.

- Option 4: Very pleasant task: This option is incorrect as the idiom emphasizes the difficulty rather than the pleasantness of a task.