The reaction described in Part A required 3.09 L of magnesium chloride. What is the concentration of this magnesium chloride solution?

Iknow the answer is a but i don't know why

If a solution is considered basic, then a) the hydroxide ion and hydronium ion concentrations are equal. b) the hydroxide ion concentration is less than the hydronium ion concentration. c) the hydronium ion concentration is greater than the hydroxide ion concentration. d) the hydroxide ion concentration is greater than the hydronium ion concentration.

What does the symbol (–hfus) indicate in a phase change?