It depends.
Which year does the letter refer to?
Does your letter specifically say it needs to be attached to your tax return, or is the letter referring to the Stock Options not being reported correctly?
If the letter is requesting a copy of your tax return, you will need to print and mail a copy of your return with the letter. Here is some information on how to print and mail your return: How to Print and Mail my Return
If the letter does not say it needs to be attached to your tax return, you should make any corrections necessary to how the Stock options were reported on your return and mail a copy of Schedule D, Form 8949 and a copy of your 1099B along with the letter to IRS.
[Edited KarenM90 3/8/2020 11:13 am]
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