Which of the following is not an element of a firm's marketing mix?

Elements of marketing4 P's.

1. Product

2. Price

3. Place and 

4. Promotion

"People" is not included with the marketing mix.

The four P's are : price, product, promotion, and place.

Hence the answer is : C people

Although PEOPLE is usually included in the5P's,6P's,7P's..., but people are not considered as part of the Marketing Mix.