Aturtle walks a mile for 50 minutes. is the unit rate when the turtle's speed is expressed in miles per hour.

The answer is:  " 3.6 mi / h " ; or, write as:  " 3 ⅗  mi / h " .

Explanation:

1 mile per 50 min.  ;  or,  1 mi./50 min. ;  

→ Convert to "mi. per hr." or, "mi./ h"
   (also abbreviated "mph" ; but let's use "mi/h" to avoid confusion.

Note:  The exact conversion:  60 min. = 1 h .

 (1 mi / 50 min) * (60 min / 1 hr) = (60/50) mi/h ;

                                 =    6/5  mi/h ;
                                               =  " 3.6 mi / h " ; or, write as:  " 3 ⅗  mi / h ".

Iam guessing c. or d.