The phrase "are most extensive" is correct and usable in written English.
It can be used to describe something that is very large in scope or range, often in a comparative context.
Example: "The research findings are most extensive, covering various aspects of the topic."
Alternatives: "are highly comprehensive" or "are very broad".
The mountain areas above 2,000 feet (610 metres) are most extensive in North Wales.
Deposits are most extensive in the Lower Mississippi Valley (LMV) and Atlantic Coastal Plain (ACP), areas presently lacking significant aeolian activity.
We find evidence for hydrated salts at all four locations in the seasons when recurring slope lineae are most extensive, which suggests that the source of hydration is recurring slope lineae activity.
Tau-related abnormalities, which include aggregations and filaments known as neurofibrillary tangles, neuropil threads, and glial tangles, are most extensive around small cerebral vessels in the frontal and temporal lobes and are prominent in the basal ganglia, brainstem, and diencephalon.
But this zone is precisely where the crevasses are most extensive, making it extremely difficult for researchers to get in and set up instruments.
The predominance and diversity of the planktonic foraminifer are related to Globotruncana aegyptiaca and Gansserina gansseri assemblages which indicate MFS 175 Ma and point to condensed sections which are most extensive in such conditions.
It is most extensive in northern Russia but continues, narrowing westward, across Sweden.
All cryopreserved specimens showed endothelial denudation that was most extensive in those subjected to rapid thawing.
In Broward County, where the election problems were most extensive, there was sheer chaos on Tuesday, the results of which caused even more turmoil today.
The ever-expanding road network of the Riau Islands is most extensive in the Riau archipelago, particularly on Batam.
