The correct answer is D) such that the vessels will be "in step". The length of the towline when towing another vessel astern should be such that the vessels are "in step", meaning that the vessels will be aligned with each other as they move through the waves. This ensures the most stable and controlled towing configuration, as the vessels will rise and fall together, reducing strain on the towline and the risk of the towed vessel broaching or capsizing. The other options are incorrect because: A) limiting the towline length to two wave lengths may not be sufficient in higher seas, B) having the towline as long as possible can make the towing configuration unstable, and C) keeping one vessel on a crest while the other is in a trough is not the optimal configuration for safe towing.