What does a symbol represents
A.Name of an element
B.One atom of element
C.Quantity element equal in mass to its atomic mass
D.All of the above

Hint:
Chemical symbols are used for the representation of any functional group, chemical elements, and compounds. it is used to identify the elements easily . It standardised the language of chemistry.

Complete step by step answer:
Chemical symbols are defined as a notation of one or two letters that represents chemical functional groups, elements and compounds. These are the abbreviations that are used in chemistry. They are not only just represented by letters but also some additional details are added in subscript and superscript for oxidation states, isotopes and isobars. Subscripts are appeared below the baseline and superscripts are appeared above the baseline.
Chemical symbol examples are:
$C$ is used for the chemical symbol carbon
$Si$ is used for the chemical symbol silicon
The symbol represents the name of the element. It is not necessary that it will start from its first letter.
Therefore, option A. Is correct
A symbol represents one atom of an element.
$H$ Represents only one atom that consists of one proton and one electron.
Therefore, option B. Is correct
The symbol represents the quantity element equal in mass to its atomic mass
Therefore, option C. Is also correct
Hence, the correct option is D. all of the above.

Note: In conclusion, symbol is the notation used to represent an element. It is not necessary that the first letter of the element will be the symbol of the element. For example sodium having a symbol $Na$
There are many functional group that have their own symbol like phenyl group represented by $Ph$ and methyl group represented by $Me$