Answer:
d. Any percentage less than 50 percent
Step-by-step explanation:
After a stock redemption, in order for an exchange to be considered "substantially disproportionate", the majority shareholder's new position must be less than the lesser between 80% of their original share of the stock and 50% (lose control of the company).
If Sam originally had 65%, 80% of his original share is:

Since 50% < 52%, Sam must own any percentage less than 50 percent for it to be treated as an exchange under the "substantially disproportionate" test.