Answer:
In sidebars
Step-by-step explanation:
In order to answer this, let's first define each of the answers given.
Caption can be defined either as a short description of the text's main idea, or as a text below an illustration, table, scheme or an image in the text providing explanatiin of its content.
Heading, basically, is the title of the part of a text, chapter, article etc.
Graph is a visual representation or comparison of a particular set of data.
Sidebar is a brief section of text, an information alongside the main text. It is physically separated from it, but providing additional information not mentioned in the main text. It is usually found in newspapers, articles, textbooks etc.
So, if the author wants to give a definition of a word, in a different manner then in the dictionary, he could use sidebars.