The term implacable denotes an inability to be appeased or pacified, characterized by relentlessness or unyielding severity. To find a synonym for implacable, we must select an option that shares a comparable meaning.
Analysis of Options:
Restrictive: Denotes limitation of access or freedom; this meaning does not correspond to implacable.
Severe: Signifies seriousness or intensity; this aligns with the relentless aspect of implacable animosity.
Amenable: Implies openness to influence or willingness to agree, which is contradictory to implacable.
Injudicious: Means unwise or lacking judgment; this is unrelated to the concept of being implacable.
Following this analysis, the word severe emerges as the most fitting synonym for implacable in this context.
Consequently, the accurate selection is: Severe