Answer C. its.
Most of the film wasn't as exciting as its beginning.
I chose C. its because the word 'their' or 'theirs' refers to one or more than one person(s) item or pet. Example: "The cat was theirs."
Example 2: "Their horse watched from the pen."
And answer D. it's, also refers to something that someone is in possession of, or describing. 'It's' is short for 'it is'.
Example: "It's a beautiful necklace."
Example 2: "It's rightfully her car."
