Final answer:
The synonym for 'inevitable' used as an adjective is 'Certain; can't avoid it'. It conveys that an event is assured to happen and cannot be avoided. Option c.
Step-by-step explanation:
The word inevitable, when used as an adjective, means something that cannot be avoided or is certain to happen. In this context, the correct synonym for inevitable would be c) Certain; can't avoid it. This is because inevitable carries the meaning that an event is assured and there is no way to prevent it from occurring.
Options a) Avoidable and b) Uncertain are antonyms of inevitable, representing things that can be prevented or are not guaranteed respectively, while d) Fortuitous is often used to describe something happening by chance, which is different from the unavoidable nature of something that is inevitable.
So Option C is the correct answer.