P:T = 2:3 = 10:15.
P:S = 5:6 = 10:12.
Now that P=10 in each ratio, the ratios can be combined:
P:T:S = 10:15:12.
We need to determine the minimum factor by which all the values in the ratio above must be multiplied.
Since T=15, and the tomato plants are planted 12 to a row, the minimum number of tomato plants must be a multiple both of 15 and of 12.
The LCM of 15 and 12 is 60.
Since 60/T = 60/15 = 4, all the values in the ratio above must be multiplied at least by a factor of 4.
Since S=12, and the spinach plants are planted 18 to a row, the minimum number of spinach plants must be a multiple both of 12 and of 18.
The LCM of 12 and 18 is 36.
Since 36/S = 36/12 = 3, all the values in the ratio above must be multiplied at least by a factor of 3.
Thus, all the values in the ratio above must be multiplied by 4*3 = 12.
12 * (10:15:12) = 120:180:144.
Thus, S=144.
correct answer D