Answer: D) A dictionary
Explanation: Etymology is the study of the origins of words and how their meanings have evolved over time. Etymological theory recognizes that words originate through a limited number of basic mechanisms, the most important of which are language change, borrowing (i.e., the adoption of "loanwords" from other languages); word formation such as derivation and compounding; and onomatopoeia and sound symbolism (i.e., the creation of imitative words such as "click" or "grunt").
While the origin of newly emerged words is often more or less transparent, it tends to become obscured through time due to sound change or semantic change. Due to sound change, it is not readily obvious that the English word set is related to the word sit (the former is originally a causative formation of the latter). It is even less obvious that bless is related to blood (the former was originally a derivative with the meaning "to mark with blood").
Semantic change may also occur. For example, the English word bead originally meant "prayer". It acquired its modern meaning through the practice of counting the recitation of prayers by using beads.