Which spectral class does a star fit into if it has a surface temperature of 20,000 K?

Answer 1

Probably a B start. I've copied the table. See below.
Temperature
O 30,000 - 60,000 K Blue stars
B 10,000 - 30,000 K Blue-white stars
A 7,500 - 10,000 K White stars
F 6,000 - 7,500 K Yellow-white stars
G 5,000 - 6,000 K Yellow stars (like the Sun)
K 3,500 - 5,000K Yellow-orange stars
M < 3,500 K Red stars

Answer 2

B