Correction: 1,500 pounds of fruits and not 1,500 of citrus fruits.
Solution:
Tangerine trees = 50 pounds/acre, t acres
Grapefruit trees = 75 pounds/acre, g acres
Total weight of fruits possible = 1,500 = 50*t + 75*g
That is,
50t + 75g = 1,500
Therefore, the first option is correct.