Answer:
The disclaimer made on May 31, 2018.
Step-by-step explanation:
IRS Section 2518(b) regarding Estate and Gift Taxes states:
"A taxpayer may make a qualified disclaimer no later than 9 months after the date on which the transfer creating the interest is made, or the date the person attains age 21."
Since Stacey's interest was created on June 1, 2019, the 9 month grace period ended on March 1, 2020. Therefore the last disclaimer made on May 31, 2020, will be subject to federal gift taxes.