Answer: C) 6 minutes.
Step-by-step explanation: to solve the given problem we can replace the given option in the values of x, and calculate y for each option, the one that results in y=1080 feet will be the correct answer:
for option A:

y=1024-1536+192-288+720
y=112 feet.
for option B:

y=3125-3750+375-450+900
y=200 feet.
for option C:

y=7776-7776+648-648+1080
y=1080 feet. As we can see the ballon will reach an elevation of 1080 feet in 6 minutes.