Given information:
- Length of closed organ pipe (L1) = 20 cm
- Fundamental frequency of L1 = Second overtone of open organ pipe (L2)

To find:
- Length of open organ pipe (L2)

Solution:
Let's first understand what is meant by fundamental frequency and overtone.

- Fundamental frequency: It is the lowest frequency produced by a vibrating object, which determines the pitch of the sound. In case of an organ pipe, it is the frequency produced when the pipe vibrates in its simplest mode, i.e. with only one antinode (point of maximum amplitude) at the open end and one node (point of zero amplitude) at the closed end.
- Overtone: It is any frequency produced by a vibrating object that is higher than the fundamental frequency. In case of an organ pipe, overtones are produced when the pipe vibrates in more complex modes, with additional antinodes and nodes along its length.

Now, let's see how the fundamental frequency and overtones are related in open and closed organ pipes.

- Open organ pipe: In an open organ pipe, both ends are open, so the air molecules at both ends can vibrate freely. The fundamental frequency of an open organ pipe is given by the formula f = v/2L, where v is the speed of sound and L is the length of the pipe. The overtones of an open organ pipe are all odd harmonics of the fundamental frequency, i.e. 3f, 5f, 7f, and so on.
- Closed organ pipe: In a closed organ pipe, one end is closed and the other end is open, so the air molecules at the closed end cannot vibrate. The fundamental frequency of a closed organ pipe is given by the formula f = v/4L, where v is the speed of sound and L is the length of the pipe. The overtones of a closed organ pipe are all even harmonics of the fundamental frequency, i.e. 2f, 4f, 6f, and so on.

Now, let's apply this knowledge to solve the given problem.

- We are given that the fundamental frequency of the closed organ pipe (L1) is equal to the second overtone of an open organ pipe (L2). This means that:

f1 = 2f2

- Using the formula for fundamental frequency of a closed organ pipe, we can write:

f1 = v/4L1

- Using the formula for second overtone of an open organ pipe, we can write:

f2 = 3f1/2 = 3v/4L2

- Substituting f1 and f2 in the above equations and simplifying, we get:

L2 = 3L1 = 3 x 20 cm = 60 cm

Therefore, the length of the open organ pipe is 60 cm, which is option (D).

Note: It is important to remember the formulas for fundamental frequency and overtones of open and closed organ pipes, and also the relationship between them. Practice solving problems using these formulas to become proficient in this topic.