That was simple, wasn't it? The symbols are there in the table, and you can see that Chlorine's symbol is Cl.
From the table, you can also see that He is the symbol for Helium. It is important that the first (and only the first) letter in an element name be capitalised. For example, 'HE' and 'he' are not valid symbol names. It must be He.
Which of the following is a valid symbol name?