The correct answer is (c).
The arithmetic mean of two numbers is the sum of the numbers divided by the number of numbers. The harmonic mean of two numbers is the reciprocal of the arithmetic mean of the reciprocals of the numbers.
The geometric mean of two numbers is the square root of the product of the numbers.
We know that the arithmetic mean of two numbers is 9 and the harmonic mean of two numbers is 4. We can use these values to find the geometric mean of the numbers as follows:
The geometric mean of two numbers is equal to the square root of the product of the numbers, which is equal to the square root of the arithmetic mean of the numbers multiplied by the harmonic mean of the numbers.
In this case, the geometric mean of the two numbers is equal to the square root of 9 multiplied by 4, which is equal to 6.
The other options are incorrect because they are not equal to the geometric mean of the two numbers.