The IRS has sent you letter
4883C because they need more information from you to verify your identity.
Unfortunately, it may take up
to 9 weeks for the IRS to process your return once you verify your identity. So
it is in your interest to do it as soon as possible.
Please read this IRS document
for more information:
https://www.irs.gov/individuals/employees/understanding-your-4883c-letter