Final answer:
The 8-letter word defined by the OED as one who assumes a false character is option (a) 'impostor', which is the correct answer out of the provided options.
Step-by-step explanation:
The 8-letter word defined by the OED as "one who assumes a false character" is impostor. Therefore, the correct answer from the given options is a. impostor. This term refers to a person who pretends to be someone else to deceive or defraud others. It is different from a deceiver, which is a broader term for someone who lies or misleads. A charlatan typically refers to a person falsely claiming to have a special knowledge or skill, while a pretender can also mean someone who claims a right or title, usually without justification.