The group of islands that forms a municipality in the province of buenos aires is called el

El Tigre

Explanation:

El Tigre is a municipality and city of the Province of Buenos Aires, in Argentina. This region is located at the mouth of the Paraná Delta, so there are numerous islets and islands. The Tigre is made up of two areas: an Insular (Delta) and a Continental. The insular territory delimits with the Lujan River, the Paraná de las Palmas, the Governor Arias canal and the Río de la Plata.

The continental territory is bounded by; to the north with the Paraná de las Palmas River and to the northeast with the Río de la Plata.

if your asking for what that says, it is

which of these words is an

but i don't know what sentence your asking for the adjective from.