The well known scientist delivered a groundbreaking lecture.
Correct: A
Option A correctly uses a hyphen to create a compound adjective ('well-known') when it precedes the noun it modifies ('scientist'). When two words act as a single modifier before a noun, they are typically hyphenated to avoid ambiguity. Options B and C misplace or incorrectly use commas. Option D incorrectly hyphenates and adds a comma.