Which of the following symbols of elements are incorrect? Give their correct symbols:
$(a)$ Cobalt $CO$
$(b)$ Carbon $c$
$(c)$ Aluminium $AL$
$(d)$ Helium $He$
$(e)$ Sodium $So$

(A, B, C, E) Incorrect,the correct symbol of cobalt is $Co$. The first letter must be capitalized and the second letter must be lowercase.
$(b)$ Incorrect,the correct symbol of carbon is $C$. Symbols consisting of a single letter must be capitalized.
$(c)$ Incorrect,the correct symbol of aluminium is $Al$. The first letter must be capitalized and the second letter must be lowercase.
$(d)$ Correct,the symbol for helium is $He$.
$(e)$ Incorrect,the correct symbol of sodium is $Na$. The symbol is derived from its Latin name,$Natrium$.