Well, because the word 'mother' is the English translation of the Latin word 'mater', they mean the same thing. So, I am assuming we are supposed to find two words which have the same etymology like this one, to complete the analogy. So, I believe the answer is A) autonomy : autos, because 'autos' is the root of the word 'autonomy', the same way 'mater' is for 'mother'.
The infinitive phrase in this sentence is to speak clearly, and it is used as a subject of the sentence. What/who is essential to being understood? To speak clearly.
