There is no option to add security questions in security section. Where can I find the option?

If you don't have the edit/reset option, then you aren't using security questions. Instead, you are using either two-step verification two-factor authentication .

See Two-step verification for Apple ID https://support.apple.com/en-us/HT204152 and

Two-factor authentication for Apple ID https://support.apple.com/en-us/HT204915  

You can edit your security question by logging into https://appleid.apple.com