Final answer:
The most commercially successful aspect of Halloween is the season for costume sales.
Step-by-step explanation:
Out of the given options, the most suitable answer is B. Season for costume sales. Halloween is known for its association with costumes, and it is the time of the year when people purchase costumes for parties, trick-or-treating, and other Halloween events. Costume sale businesses experience a significant increase in sales and profits during the Halloween season.