Third-person omniscient narrators are likely to be reliable because . they're impersonal and know everything about the storyb. they don't care enough about the characters to liec. none of the aboved. both “a” and “b”

Third-person omniscient narrators are likely to be reliable because A. they're impersonal and know everything about the story.
If somebody is omniscient, it means that they know everything, therefore they can be trusted with such knowledge.