The correct answer is D. To link ideas across sentences and paragraphs
Explanation:
In texts including essays, transitional words are necessary because they show the relationship between different elements including sentences and paragraphs and therefore help the author to appropriately link ideas in the text. Examples of transitional words include the words therefore that shows consequence, firstly/ secondly that show order, in addition that is used to add ideas or for instance that is used to add examples. The previous ideas imply by using transitional words the author can show the connection between elements in a text and therefore the purpose of transitional words is "to link ideas across sentences and paragraphs".
