Fifty miles __ not a long distance.

Explanation: Fifty miles is a long distance.
এখানে Expressions of time and distance are treated as single entities so they don’t take plural verbs: twenty miles, six days, two minutes, four meters etc - all take singular verbs.
 
সুতরাং সঠিক উত্তরটি হবে – Fifty miles is not a long distance.
 
SOURCE: Advanced Learners Communicative English Grammar and Composition With Solution (For Class 9-10)