The correct answer is "grammar"
Linguist is a person who studies languages, semantics is the science of word and sentence meaning, and arbitrary means that something is subjected to individual will or judgement.
The difference between the words swarmed and assembled should give you a clue as to what the answer is. Swarmed means to move somewhere in large numbers, usually crowding around something of interest to get a closer look. Assembled means to gather together in one place for a particular reason. This is usually done in an orderly fashion.
Most dictionaries divide syllables with a dot or a space. The correct answer is D, dot or space.
Answer is B to my knowledge