Hi there!
Voltage in a series can be expressed by the following:
[tex]V_T = V_1 + V_2[/tex]
In words, the total voltage is equal to the sum of the individual voltage drops in a SERIES circuit.
We can solve for the total voltage:
[tex]V_T = 10 + 10 = \boxed{\text{ D. 20 V}}[/tex]