Since we are mixing the same solution we can apply,

Solution:

Given:

Molarity of 1st solution (NaOH) = 1M

Volume of 1st solution (NaOH) = 2.5 litres

Molarity of 2nd solution (NaOH) = 0.5M

Volume of 2nd solution (NaOH) = 3 litres

To find: Molarity of resultant solution

Step 1: Calculation of moles of NaOH in 1st solution

Moles of NaOH = Molarity of solution x Volume of solution

Moles of NaOH in 1st solution = 1M x 2.5 litres = 2.5 moles

Step 2: Calculation of moles of NaOH in 2nd solution

Moles of NaOH in 2nd solution = 0.5M x 3 litres = 1.5 moles

Step 3: Calculation of total moles of NaOH in the mixture

Total moles of NaOH = Moles of NaOH in 1st solution + Moles of NaOH in 2nd solution

Total moles of NaOH = 2.5 moles + 1.5 moles = 4 moles

Step 4: Calculation of total volume of mixture

Total volume of mixture = Volume of 1st solution + Volume of 2nd solution

Total volume of mixture = 2.5 litres + 3 litres = 5.5 litres

Step 5: Calculation of molarity of resultant solution

Molarity of resultant solution = Total moles of NaOH / Total volume of mixture

Molarity of resultant solution = 4 moles / 5.5 litres = 0.73M

Therefore, the molarity of resultant solution is option C (0.73M).

Conclusion:

The molarity of resultant solution is 0.73M, which is obtained by mixing 1M and 2.5 litre NaOH solution with 0.5M and 3 litre NaOH solution.