Final answer:
The best definition of stagnant is a lack of growth or progress. In the context of the sentence, it refers to the social media influencer's lack of growth in followers and brand deals.
Step-by-step explanation:
The best definition of stagnant based on the context provided in the sentence is that it means a lack of growth or progress. In the given sentence, it refers to the lack of growth in the social media influencer's followers and brand deals despite her best efforts. The word 'stagnant' can be used to describe a situation or condition that is not changing or progressing.