Six-mile radius or six-miles' radius????
locations within a __________ radius of London’s Charing Cross railway station.
A) six- mile
B) six miles'
Tell me, please, which option is correct and why. I thought "six mile" was wrong but my teacher told me it was the right answer. And I wanna hear from the native speaker which one is really correct!)

Question

Six-mile radius or six-miles' radius????
locations within a __________ radius of London’s Charing Cross railway station.
A) six- mile 
B) six miles' 
Tell me, please, which option is correct and why. I thought "six mile" was wrong but my teacher told me it was the right answer. And I wanna hear from the native speaker which one is really correct!)

Answer

Correction:  "I wanna hear" is not standard written English.  Use "I want to hear."

"six-mile" radius is the answer for English exams.

The basic rule is to hypenate when a modifier refers to another modifier.
This is an expensive, old car.  (expensive modifies car and old modifies car)
This is a 50-year-old car.  (neither 50 nor year modify car)

More information here:
https://www.quickanddirtytips.com/education/grammar/stacked-modifiers

Answer

definitely six mile radius
No need for the apostrophe.

Answer

Hi Olena. The 'six-mile' is acting as an adjective in this sentence. We don't make adjectives plural in English.  It was a difficult 26-mile race. -- Both difficult and 26-mile are describing race, so both will remain singular. Hope this helps, Ellen