**Given Information:**

- The fundamental frequency (f1) of an open organ pipe is equal to the third harmonic (f3) of a closed organ pipe.
- The length of the closed organ pipe is 20 cm.

**To Find:**

The length of the open organ pipe.

**Solution:**

**Understanding Harmonics:**

In a closed organ pipe, only odd harmonics are present. So, if the fundamental frequency is f1, then the frequencies of the harmonics are:

f1, f3, f5, f7, ...

In an open organ pipe, both odd and even harmonics are present. So, if the fundamental frequency is f1, then the frequencies of the harmonics are:

f1, f2, f3, f4, f5, f6, ...

**Relationship Between Frequency and Length:**

The frequency of a pipe is inversely proportional to its length. Therefore, we can write the following relationship:

f1 (closed) ∝ 1/L (closed) -- (1)

f1 (open) ∝ 1/L (open) -- (2)

**Given Relationship:**

According to the given information, the fundamental frequency of the open organ pipe (f1, open) is equal to the third harmonic of the closed organ pipe (f3, closed).

f1 (open) = f3 (closed)

**Applying the Relationship:**

Substituting the relationships (1) and (2) into the given equation, we get:

1/L (open) = 3/L (closed)

**Simplifying the Equation:**

Cross-multiplying the equation, we have:

L (open) = 3 * L (closed)

**Substituting the Given Value:**

Given that the length of the closed organ pipe is 20 cm, we can substitute this value into the equation:

L (open) = 3 * 20 cm

L (open) = 60 cm

**Converting to Meters:**

The length of the open organ pipe is given in centimeters, but it is customary to work with SI units (meters). Therefore, we need to convert the length into meters:

L (open) = 60 cm * (1 m / 100 cm)

L (open) = 0.6 m

**The Final Answer:**

Therefore, the length of the open organ pipe is 0.6 meters or 60 cm.

The correct answer is not provided in the options given.