The answer is 5.
The missing number is the sum of the other two numbers in the series. The first number is 1, the second number is 3, and the third number is 5. The sum of the first two numbers is 4, and the sum of the last two numbers is 8. The missing number is the number that, when added to the first two numbers, gives the same sum as when it is added to the last two numbers. The only number that satisfies this condition is 5.
Option A: 3 is not the missing number because it is not the sum of the other two numbers in the series. The sum of the first two numbers is 4, and the sum of the last two numbers is 8. 3 is not a number that, when added to the first two numbers, gives the same sum as when it is added to the last two numbers.
Option B: 1 is not the missing number because it is not the sum of the other two numbers in the series. The sum of the first two numbers is 4, and the sum of the last two numbers is 8. 1 is not a number that, when added to the first two numbers, gives the same sum as when it is added to the last two numbers.
Option C: 5 is the missing number because it is the sum of the other two numbers in the series. The sum of the first two numbers is 4, and the sum of the last two numbers is 8. 5 is a number that, when added to the first two numbers, gives the same sum as when it is added to the last two numbers.
Option D: 2 is not the missing number because it is not the sum of the other two numbers in the series. The sum of the first two numbers is 4, and the sum of the last two numbers is 8. 2 is not a number that, when added to the first two numbers, gives the same sum as when it is added to the last two numbers.