Exercise of an Incentive Stock Option Under Section 422(b). Use Form 3921 Copy A to print and mail payment information to the IRS. 3921 forms are printed in a 3-up format, on 8 1/2'' x 11'' paper with 1/2'' side perforation, and are printed on 20# laser paper. Mail as a batch to the IRS. Order by number of forms, not sheets.