a) 4(g - 10) - it represents the gas she paid for.
Since the total number of gallons she used to fill the tank is unknown, so 4 in 4(g-10) represents the cost per gallon. As g-10 is an unknown variable and only 4 is known value that's why 4 is representing cost.
b) g represents the total u number of gallons she used to fill the tank.
c) 10 represents the number of free gallons she received as a gift.