Question

When California applied for statehood, Southern states feared...
they would lose equality in the Senate.
enslaved workers would escape.
they would have to pass new laws to maintain slavery.
the gold-mining industry would overtake agriculture.

Answer 1

They feared that they would lose equality in the Senate, that they would have more free union states than slave states. Hope this helps!

Answer 2

The correct answer is:  

They would lose equality in the Senate.

Since the Senate had just approved the admission as a free U.S. territory of Iowa in 1846 and Oregon, Wisconsin in 1848, the numerical equality between free and slave states in the Senate had just been restored.  

Southern senators wanted the next state admitted to the Union to support slavery because they feared they would lose equality in the Senate.