The word structure refers to ‘Components or parts of words’ i.e. option ‘c’
Explanation :
Samuel Johnson is seen performing the work of a lexicographer, compiling a resourceful dictionary for the users to refer to. The structure refers to the parts or components of the words which help the arrangement of words in the dictionary in such a way that it makes the reader locate the word and understand its composition and origin.
The components can be explained like, for instance, like many words in the English language are constituted by adding prefixes, suffixes or root words.