Like adjunct and addition, both are defined as 'to add', so they are synonyms.
In the choices, the best answer would be c. straits : hardship.
Abstentions is to abstinate, absence is not being present
Commodiousness is having a lot of space, size is a fixed proportion
Conjugation is verb changes, nouns are simply names
Straits is something that causes distress, hardship are challenged