The phrase "a most extensive" is correct and usable in written English.
It can be used to emphasize the degree or extent of something, often in a formal or literary context.
Example: "The report provided a most extensive analysis of the environmental impacts of the project."
Alternatives: "a very extensive" or "an extremely extensive".
That corporation was expected to engage in, and did engage in, a most extensive ocean transportation business, and its ships were seen in every great port of the world open during the war.
The victim is Mr Dadd, sen., 55 years of age, a person who a few years ago carried on a most extensive business at Rochester, as a chemist and druggist.
Recently deceased President George H.W. Bush was especially fond of foreign affairs and had a most extensive resume in public service prior to becoming president in 1989.
It commands a most extensive prospect over the surrounding country, and from it can be seen no less than twelve other fortified positions.
He carried on a most extensive correspondence with just about all the literary and political personalities of the day, and for many years also provided a meeting place for them at his country estate in Pempelfort, by hosting there a very popular literary salon.
The Rhine... lay revealed before us for many a league, twisting and twining like a serpent of silver... dotted with innumerable islands, and flowing through a most extensive plain, perfectly flat.
A petroleum engineer who spent the early 1990s running the King Ranch, Jarvis started Spinnaker with a library the most extensive collection of 3-D seismic data for the Gulf of Mexico ever assembled.
Writing in 1774, Joseph Priestley, a leading Unitarian and the foremost scientist in Britain, wrote that the Observations "contains a new and most extensive science" and promised that "the study of it … will be like entering upon a new world".
Their manufacturing establishments an among the most extensive workshops in the country.
I have only to assume that it's some kind of a fanatical reaction or slavish obedience to a contingency plan". In the most extensive U.S. air action over the gulf, an American A-6 Intruder bomber, launched from the carrier Enterprise in the northern Arabian Sea just outside the gulf, destroyed one Iranian speedboat and badly damaged the other two.
But of this season, the already impressive property just undertook a substantial renovation, the most extensive since it opened a decade ago - the owners call it "the Reimagine Project". All eighteen of the one to five-bedroom residences and penthouses has been restyled and redesigned.
