Final answer:
The lake in Quebec, Vermont, and New York called b. Lake Champlain once formed part of an ancient sea. The Quebec Act's provision related to extending Quebec's boundaries into the Ohio Valley, as indicated in option b of the resources provided.
Step-by-step explanation:
The name of the lake in Quebec, Vermont, and New York that fills a broad valley and was once its own sea is b) Lake Champlain. This lake offers a vital historical context as it was indeed a remnant of a past sea, influencing the region's ecological and geographical landscape.
The Quebec Act provision is unrelated to the name of the lake itself. However, as your reference point, one of the provisions of the Quebec Act included the extension of the boundaries of Quebec into the Ohio Valley, which is reflected in answer b. The boundaries of Quebec were extended into the Ohio Valley mentioned in your provided resources.