Create a function with the signature:

bool isvalidoption$($ char $)$;

The function accepts the following characters as Valid: $\text{'}$a$\text{'},\text{'}$b$\text{'},\text{'}$c$\text{'},\text{'}$A$\text{'},\text{'}$B$\text{'},\text{'}$C$\text{'}$ and returns 'true'; otherwise 'false' is returned.

RUN TEST CASES for values:

$\text{'}$a$\text{'},\text{'}$b$\text{'},\text{'}$c$\text{'},\text{'}$A$\text{'},\text{'}$B$\text{'},\text{'}$C$\text{'},\text{'}$d$\text{'},\text{'}$E$\text{'}.$