Conversion questions are best dealt using a one step approach.
Let me illustrate: Let's say we want to convert a speed of 100 miles/hour to feet/sec.

Speed = 100 miles/1 hour = (100 x 5280 feet)/(1 x 3600 sec)= 146.67 feet/sec

So, simply convert the units to what you want along with the conversion factor.

Let's take your question now.
If an object travels 100 feet in 2 seconds, what is the object’s approximate speed in miles per hour? (Note: 1 mile = 5280 feet)

Speed = 100 feet/2 sec = (100×1/5280 miles)/(2×1/3600 hour) = 5x3600 miles/528 hr = 34 miles/hr

Note: Since 1 mile = 5280 feet, hence here to convert we say 1 foot = 1/5280 miles.

Answer: C