How will you prepare 250 ml of TE Buffer using 0.5 M EDTA and 1M Tris-Cl stock?
Since molar concentrations are an expression of the molecular weight per liter, the fact that the question asks for 100 ml is irrelevant. Whether you are dealing with 1000 ml, 100 ml or 10 ml, the Molar concentration will be the same. What is important is the change from 10x to 1x.
Since the concentration, 10x is divided by 10 to arrive at a 1x concentration, then the Molar concentration is also divided by 10.
The concentration of Tris-borate in 100 ml of 1x TBE is 0.089 M