Final answer:
The correct answer is 'a) Amicable', which is derived by adding the letter 'c' to 'amiable' and means friendly and agreeable.
Step-by-step explanation:
The question asks about a word that when added a single letter to "amiable" means the same thing. Among the given options, the correct answer is a) Amicable. By adding the letter 'c' to "amiable", you form the word "amicable", which also means friendly and agreeable in disposition. It is an adjective often used to describe peaceful, harmonious relations especially in situations where conflict could be expected, such as 'amicable divorce'. The other options do not represent a word that would mean the same as "amiable" when a letter is added to it.