No publicly traded company reached a $1 trillion market capitalization in the 20th century.
The first widely recognized public-company crossing of the $1 trillion threshold came in 2018, when Apple reached that valuation. The 20th century ended on December 31, 2000, so the milestone occurred almost two decades later rather than during the era of early personal-computer companies.
Market capitalization rose dramatically in the 21st century as large technology firms expanded worldwide and investors placed higher valuations on software, platforms, digital advertising, cloud services, and consumer electronics. Earlier corporations could still be extremely large, but none reached the modern trillion-dollar market-cap threshold.
This question uses “none” as the single correct answer because the historical record has no qualifying company. It is also important not to confuse a company’s market cap with inflation-adjusted historical value, annual sales, or the total value of an entire stock exchange.