It is true that a writer begins a new paragraph when he changes his focus. Option A is correct.
A paragraph is a collection of related sentences dealing with a single topic.
You should start a new paragraph when:
You will introduce a new idea or point. New ideas should always start in new paragraphs.
To contrast information or ideas. To contrast sides in a debate you should use different paragraphs, as well as if you add different points in an argument, or any other difference.
When your readers need a pause.
When you are ending your introduction or starting your conclusion. Your introductory and concluding material should always be in a new paragraph.