Answer:
The LEAST effective substitution for the word ’calm’ in the sentence labeled (2) is "quiet"
Explanation:
The least effective substitution from the given options is quiet, due to its simple meaning about the intention of the full sentence "His calm performances encouraged the Marines to adopt seven other wild horses." this sentence talks about the general behavior of the horse and quiet mostly refers to the noise that is produced, while the other has a wider use depending on the context.