When towing another vessel astern, the length of the towline should be __________.

Correct Answer: Option D — such that the vessels will be "in step"

Maintaining the tow 'in step' is a fundamental principle of seamanship. By ensuring both vessels rise and fall with the waves in unison, the master minimizes extreme tension fluctuations on the hawser, thereby protecting the towing equipment and ensuring a safer, more controlled transit.