Answer:
The correct option is C.) AB = 5
Explanation:
the diagram of given triangle is shown in figure-1
M, N and O correspond to A , B and C respectively.
and also given that triangle MNO is 0.5 times of triangle ABC
so, MN =(0.5)AB , NO = (0.5)BC and MO =(0.5)AC
then 
 MO =(0.5)AC
 Put OM = 5
 5 =(0.5)AC
 Divide both the sides by 0.5,
 

 10 = AC
 If AB = x and AC = 2x
 AC = 2x
 10 = 2x
 

 5 = x
since AB = x the AB = 5
 hence, correct option is C.) AB = 5