Answer:
d. Symbols.
Explanation:
Symbols are the abbreviations used to represent a particular thing. Similarly, chemical symbols are representative of the elements they denote based on their names.
The letters C stands for Carbon, Al for Aluminum, and Au for Gold. While it is easy to figure out the letters/ symbols and elements from the words that are their names, gold's symbol "Au" comes from the word "Aurum" which means yellow.
Thus, the above-given letters are examples of symbols.
