The isthmus of panama is located at number on the map above.

a. 1
b. 2
c. 4
d. 5

D. 5, as given in the figure.

Explanation:

The isthmus of Panama is a narrow strip of the landmass that is found in between the Caribbean and the pacific sea, which connects the north and the South America.  And has the country of Panama and the Panama canal thus forms an isthmus and this term was first given in the 1910. And has been formed around 2.8 million years ago. Geologically when the earth's crust slowly collided it forced the cocos plate to go under the Caribbean plate. Their collision leads to the formation of the underwater volcanoes and thus enough to form an island and that resulted from the expansion of the sediments that got deported in the area. The formation of this plays a major role in the biodiversity and thus is an essential bridge to the planet.

I'm pretty sure it's c. extremely gradual but not 100% sure though.

Apaid their profits to high officials