Final answer:
The common nouns in the given sentence are 'group' and 'musicians', making the correct answer d. group, musicians.
Step-by-step explanation:
The common noun(s) in the sentence "I jammed with a group of Emma Pillsbury's favorite musicians." are "group" and "musicians".
Here, 'group' is a collective noun referring to a number of individuals that can be considered as a single unit or multiple individuals acting separately.
On the other hand, 'Emma Pillsbury' is a proper noun as it signifies a specific name, and is therefore not a common noun. Similarly, 'jammed' and 'favorite' are not nouns at all; they are a verb and an adjective respectively.
Therefore, the correct answer is d. group, musicians.