Final answer:
The proposed assumption is that the extent of attendance in professional media forums is not negligible. This assumption is supported by the information provided.
Step-by-step explanation:
The proposed assumption in the question is that the extent of attendance in professional media forums is not negligible. Based on the information provided, it can be inferred that social media engagement is a substantial feature of brand marketing plans. The fact that about 330 marketing professionals attended the Media Innovations Forum indicates the importance and relevance of professional media forums to the industry.
Therefore, the correct answer is A. Assumption made.