You are given a measurement in feet and asked to compare that to measurements in inches. It is convenient to convert the measurement in feet to a measurement in inches to make the comparison easier. (There are other ways to do the comparison, but this way may be the least work.)
We need to start with a conversion factor.
1 foot = 12 inches
Multiplying by 4 1/4 gives
4 1/4 feet = (48 + 3) inches = 51 inches
The set of measurements that are all less than 51 inches is
... B. 50 inches, 48 inches, 49 inches