First, we need to find how many acres there are. (0.75)(16) = 12 acres, and (1.5)(12) = 18 acres, so there are 30 acres in total.

Each worker can mow 0.5 acres in 20 minutes, so we can say: 0.5 acres every 20 minutes, or 0.5/20. This means, after one hour, one landscaper has mowed 3(0.5)/3(20) = 1.5 acres/60 minutes = 1.5 acres/1 hour. In 6 hours, the landscaper would have mowed 9 acres: (1.5)(6)/1(6) = 9 acres/6 hours.

So if one worker can mow 9 acres in 6 hours, then two would mow 18 acres in 6 hours, three would mow 27 acres in 6 hours, and four would mow 36 acres in 6 hours. However, we only have 30 acres. Four workers can mow more than 30 acres in 6 hours, so they can certainly do 30 acres within 6 hours (in fact, it would take them 5 hours).

Sure, it's true that five workers, six workers, or any number of workers greater than or equal to four workers can complete the job within 6 hours, but were looking for the minimum number of workers. That minimum number is 4 workers.

The answer then is C.