Root words are demonstrated as the basic words that assists in deriving new words by addition of suffixes at the end or prefix prior to the root word. It is described as the most basic meaning associated with a word that assists the readers to understand the meaning of its derived forms.
As per the question, 'kilometer' is the word from the sentence that involves an appropriately underlined root 'meter' which means 'measure'(derived from Greek). In the given word, 'kilo-' is the prefix implying 'a unit of measuring length equivalent to 1000 meters as per International System of units'. Thus, the root word 'meter' and prefix 'kilo' together comprises the word 'kilometer' meaning 'a measuring SI unit of length'. This root word 'meter' is basically employed to form various derivatives of measuring units through addition of different prefixes. Therefore, <u>option D</u> is the correct answer.
