Final answer:
The motivation of a threat actor clandestinely extracting proprietary data for black market sale aligns with a financial gain objective. Other motivations like political activism, personal vendetta, and corporate espionage are less likely considering the financial context provided.
Step-by-step explanation:
The objective of a threat actor who infiltrates an organization's network to silently extract sensitive proprietary data that has high value on the black market aligns most likely with the motivation of financial gain. While motivations such as political activism, personal vendetta, and corporate espionage could potentially drive such actions, the mention of the black market indicates a financial incentive. This activity is indicative of cybercrimes that are often associated with actors seeking monetary benefit through the sale of stolen data, engaging in identity theft, and financial theft, significantly impacting businesses, organizations, and individual security.