To convert 221 seconds to minutes, which ratio could you multiply by? (look at image! )

221 seconds to x minutes.

To convert the seconds into minutes, you'd need to multiply by something that would cancel out the seconds. So if the 221 seconds are the numerator right now, you'd have to multiply by something with the seconds in the denominator. 

The only one here that makes sense is
 D. 1 minute/60 seconds