It is essential for historians to know how to a. write. b. spell. c. give a speech. d. do complex math.

 it is essential for historians to know how to "write," since almost all historians will have to write a thesis of some sort at one point or another in their career. 

Hello.

The answer is 

Write.

The have to write becasue if its gonna go down in history people have to read it. They would be died so the speech and math wouldnt matter.

Have a nice day

The led the formation to the united states