Solution: The third proportional to two given numbers is the number which is in the same ratio as the other two numbers. This number is found by solving a particular equation.

Definition: The third proportional to two given numbers is a number which is in the same ratio as the other two numbers.

Formula: The formula for finding the third proportional to two given numbers a and b is given by:

a:b = c:x

where x is the third proportional to a and b.

In this question, the two given numbers are 1 and 2.

Therefore, the equation becomes:

1:2 = c:x

Here, c is a constant and x is the third proportional to 1 and 2.

Solving the equation, we get:

1x = 2c

x = 2c

Since c is a constant,

x = 2

Therefore, the third proportional to 1 and 2 is 4.

Hence, the correct answer is Option D. 4.

If you think the solution is wrong then please provide your own solution below in the comments section .