Final answer:
A proper lookout must be maintained at all times according to Rule 5 of the COLREGs, to assess the situation at sea and prevent collisions or other hazards. So, the correct answer is option a.
Step-by-step explanation:
The question refers to the rules of navigation concerning the maintenance of a proper lookout while at sea. According to both International and Inland navigation rules (Rule 5 of the COLREGs), a proper lookout must be maintained at all times. This is necessary to ensure the safety of the vessel and to make a full appraisal of the situation and of the risk of collision, stranding and other dangers to navigation. So, the correct answer is option a.