For a closed pipe, only odd harmonics are present. first overtone frequency is 3f\(_o\), 2\(^{nd}\) overtone frequency is 5f\(_o\) etc.

4\(^{th}\) overtone frequency = 9f\(_o\)

900 =  9f\(_o\)

f\(_o\) = \(\frac{900}{9}\) = 100Hz