Rationale
Complacent and agreeable are synonyms.
In the English language, both words are often used to describe someone who is satisfied with a situation and does not want to change or challenge it. They both express a sense of contentment and easy acceptance.
A) Perceptive - obtuse
'Perceptive' refers to an individual who is observant and has a keen understanding, while 'obtuse' refers to someone who is not quick or alert in perception, feeling, or intellect; it is not sharp, acute, or pointed. Therefore, these two words are antonyms, not synonyms.
B) Conscientious - careless
'Conscientious' describes someone who is diligent and careful, while 'careless' refers to a lack of attention or consideration, resulting in error or damage. These two words also represent opposite meanings, making them antonyms rather than synonyms.
C) Complacent - agreeable
Both 'complacent' and 'agreeable' can describe someone who is easily satisfied or willing to accept something. They both suggest a person who is not inclined to argue or make a fuss, and thus, they are synonyms.
D) Submissive - aggressive
'Submissive' describes someone who is ready to conform to the authority or will of others, while 'aggressive' refers to someone who is ready to attack or confront. These words have contrasting meanings and are, therefore, antonyms.
Conclusion
Synonyms are words that have the same or similar meanings. In this context, the pair 'complacent' and 'agreeable' represents synonyms as they both convey a sense of easy acceptance or contentment. The other pairs provided (perceptive - obtuse, conscientious - careless, and submissive - aggressive) are antonyms, representing opposite or contrasting meanings, rather than synonyms.