Answer: Option (a) is correct.
Explanation:
(a) The equilibrium can be restored if Exxon gas station reduces the prices. This change will achieve equilibrium.
(b) If the shell gas station reduces the prices, then the gap between the price charged by Exxon gas station and shell gas station become larger. So, this change will not be able to achieve equilibrium.
(c) If Exxon gas station increases the prices, this will also results in larger gap between price charged by Exxon gas station and shell gas station. So, this change will not be able to achieve equilibrium.
(d) If both stations reduce their prices by 50 cents per gallon, the gap remains the same. So, this change will not be able to achieve equilibrium.