The negotiations were marred ____allegations of corruption that cast a shadow_____ the legitimacy of the entire process.
The negotiations were marred ____allegations of corruption that cast a shadow_____ the legitimacy of the entire process.
Explanation
The negotiations were marred amid allegations of corruption that cast a shadow under the legitimacy of the entire process."
Amid is used to indicate the situation in which something happens,
Under suggests something affecting the legitimacy of the process.