The meaning of "sold at option" [closed]

Solution 1:

This is referencing stock options:

A stock option gives an investor the right, but not the obligation, to buy or sell a stock at an agreed upon price and date.

So I read your quote:

AAA, sold 20% of their company,and the other 80% could be sold at option in 2021

to mean that when AAA sold their company they relinquished %20 of their stock immediately. They also provided the investor with the option to purchase the remaining 80% of the company in 2021 at a set price. The excerpt is saying that this option could be exercised by the investors in this deal, which is expressed here as "sold at option".