Pre-voyage planning for a towing vessel operating seaward of the U.S. territorial sea baseline requires the master to check the planned route for proximity to hazards before departure. If a substantial deviation from the planned route becomes necessary during the voyage, when must the new route be planned?

Why A is correct

33 CFR §164.80(c)(3) states that if a decision is made to deviate substantially from the planned route during a voyage, the master or mate must plan the new route before deviating from the planned route. This requirement applies when any part of the intended voyage is seaward of the U.S. territorial sea baseline. Accurate application of compass corrections (T-V-M-D-C) to the replanned route is part of the navigational competency required by 33 CFR §164.78(a)(5).