C20 is an ERC20 token so it can only be stored at an Ethereum wallet. You should use a wallet where you control the private keys and can interact with the tokens easily.
3. MyEtherWallet or MyCrypto; follow the prompts and store your keys securely. When using these web interfaces ensure that you are at the correct website with a valid SSL certificate. These sites are common targets for phishing.
4. Another client such as Parity, Coinomi, Mist, etc. With applications of this kind, you are relying on a third party. Use of these applications depends on how comfortable you are with third parties potentially accessing your keys and being capable of exerting a degree of control over your funds. Some compare this to storing tokens on an exchange long-term.