The correct answer is D) The gym was full of people trying to fulfill their New Year's resolutions: losing weight, running five miles each week, and lifting weights regularly.
Step-by-step explanation:
One of the recommendations for sentences that include a list or set of items with the same importance is that all related items are included using the same grammatical structure or pattern, which is known as parallelism.
This is missing in the sentence "The gym was full of people trying to fulfill their New Year's resolutions: losing weight, to run five miles each week, and lift weights regularly" because the New Year's resolutions are included using different patterns, for example, "losing weight" includes an -ing that cannot be found in others. Due to this, the strongest revision of the sentence is one that presents the list using the same pattern, which occurs in option D as all elements are introduced using -ing "losing weight, running five miles each week, and lifting weights regularly".