Final answer:
The unextended due date for a calendar-year C corporation tax return for 2018 is c. April 15.
Step-by-step explanation:
The unextended due date of the tax return of a calendar-year C corporation for 2018 is c. April 15. This is the deadline for most C corporations to file their tax return for the previous calendar year. If a C corporation requests an extension, the extended due date would be October 15.
It's important for corporations to meet the tax filing deadline to avoid penalties and interest charges. Corporations should consult with their tax advisor to ensure they meet all applicable filing requirements.