Given a class with the following private variable, how would you code a read$-$only expression$-$bodied indexer?

private List products;

a$)$ public Product get $=>$ products$[$i$]$;

b$)$ public Product get$[$this$]=>$ products$[$i$]$;

c$)$ public Product this$[$int i$]=>$ products$[$i$]$;

d$)$ public Product this$[$i$]=>$ products$[$i$]$;