Final answer:
The '3rd Sub flag' possibly refers to the third substitute flag in maritime signal flag communication or the 'Bravo' flag in the International Code of Signals, none of which correspond exactly to the options provided in the question.
Step-by-step explanation:
The term 3rd Sub flag refers to a signal flag used in maritime contexts. Specifically, it designates the B flag in the International Code of Signals, which stands for 'Bravo'. However, when you mention 3rd Sub flag, it could also imply the third substitute flag used to repeat the first signal flag in a sequence to avoid confusion with other signals. Neither of these has an assigned meaning definitively corresponding to options A, B, C, or D in the question.
To understand maritime communication, it's important to know that signal flags are used to convey messages between ships about their operations or situations. Each flag corresponds to a letter of the alphabet and often has a special meaning when flown individually. For precise meanings, sailors refer to the International Code of Signals publication, which standardized communication for ships of different nationalities.