Since it is given that Boris takes x minutes to run 5 miles it means that Boris' speed will be given as: .
Now, if Dylan runs slower than Boris and takes 12 more minutes to complete the 5 miles run, then that would mean that Dylan takes (x+12) minutes to complete the same run.
In this case we can see the Dylan's speed will be .
Thus, the denominator represents the time Dylan takes to run 5 miles if he takes 12 minutes more than Boris.
Therefore, out of the given options, the second option is the correct option.