California finally joined the Union following the death of

O President Millard Fillmore.
O President Zachary Taylor.
O Senator William Seward.
Senator John C. Calhoun.

Zachary Taylor

California joined the Union following the death of Zachary Taylor. It was previously owned by Mexico. William Seward was a secretary for Lincoln, it is not the answer. Fillmore was not elected during this time, so it is not him, and Calhoun was a senator.

Hope this helps you!

they thoght they were going to nuke each other

the answer is