The answer is B. This is because Sodium has 1 valence electron and Fluorine has 7 valence electrons. All elements want 8 valence electrons so they may be stale, like the noble gases are. Hope this helps.
The molarity of a solution is defined as the moles of solute over liters of solution. That means that M=5/2=2.5. Therefore the answer is 2.5M. I hope this helps. Let me know if anything is unclear.
