Final answer:
The concentration of the unknown NaOH solution is (a) 0.7617 M.
Step-by-step explanation:
To find the concentration (M) of the unknown NaOH solution, we can use the equation:
 volume of HCl solution (L) x molarity of HCl = volume of NaOH solution (L) x molarity of NaOH 
First, convert the volumes to liters:
39.88 mL = 0.03988 L 
46.08 mL = 0.04608 L 
Next, plug in the values into the equation: 
0.04608 L x 0.6592 M = 0.03988 L x concentration (M)
 Solve for the concentration:
 concentration (M) = (0.04608 L x 0.6592 M) / 0.03988 L
 concentration (M) = 0.7617 M