answer: to tie one paragraph to aanother, coherence (a and c)
explanation: when something has coherence, all of its parts fit together well. an argument with coherence is logical and complete, same for an essay. transition words are also used to not confuse the reader i guess. instead of making a new paragraph (similar to the one you just wrote about) you can use a transition word to connect them. sorry if my explanation is a bit off but i'm sure you can find better understandings elsewhere. hope i helped out a bit. good luck on your work.