Final answer:
The root word in 'dictionary' means 'to speak', relating to the Latin word 'dictio' for 'saying' or 'speech'.
Step-by-step explanation:
The meaning of the root word underlined in the word dictionary is 'to speak'. This root is derived from the Latin word 'dictio' meaning 'saying' or 'speech'. Hence, the correct answer is B. 'to speak'.