Which of these is the most likely impact of solar prominence on earth?
a) rerouting of aircraft
b) increase in severity of hurricanes
c) long-term electrical power generation
d) improvement in satellite communication

Solar prominences themselves are of no concern because they are visible in the Hydrogen Alpha wavelength. They are anchored in place by magnetic fields. When these fields break or reconnect, it can send the plasma that makes up the prominence away from the sun. If one of these clouds impacts Earth, they are called CMEs or coronal mass ejections. Depending on the magnetic orientation of the cloud with respect to Earth's the CME can break down our magnetic field resulting in geomagnetic storms, aurorae, power grid fluctuations, and particle radiation near the poles, satellite single upset events, and radio blackouts. 

Thus, letter a is the answer. 

the old price per ounce is c/4.

the new price per ounce is c/3.6.

the ratio of the old price to the new one is

(c/4)/(c/3.6) = (c/c)×(3.6/4) = 0.9 . . = 9/10 . . "nine tenths" in either form