Which of the following is the most standard formal way to close a business letter?

In formal business communication, 'Sincerely' is the gold standard for general professional correspondence, while 'Respectfully' is the standard for formal letters addressed to superiors, government officials, or high-ranking dignitaries. Both are considered formally correct, whereas 'Regards' is generally categorized as semi-formal or informal.

Step-by-step Derivation:
Step 1: Analyze Option A ('Regards'). 'Regards' or 'Kind regards' is commonly used in emails and semi-formal business correspondence, but it lacks the formal weight required for a traditional formal business letter.
Step 2: Analyze Option B ('Sincerely'). 'Sincerely' (or 'Yours sincerely') is the universally accepted standard for formal business letters where the recipient's name is known.
Step 3: Analyze Option C ('Respectfully'). 'Respectfully' (or 'Yours respectfully') is the standard formal closing used when writing to a person of higher rank, a government official, or in highly formal institutional contexts.
Step 4: Compare the options. Since both 'Sincerely' and 'Respectfully' are recognized as standard formal closings depending on the specific hierarchy and context of the business relationship, Option D is the most comprehensive and correct choice.