The question revolves around understanding ovarian tumors, specifically thecoma-fibroma, and their associated conditions. Let's analyze the options given and determine why the correct answer is "Endometrial hyperplasia is often associated."

Explanation of Thecoma-Fibroma:

Thecoma-fibroma group of tumors is a type of stromal tumor of the ovary, and it has characteristics distinct from other ovarian tumors like germ cell tumors or epithelial tumors. These tumors are derived from ovarian stromal cells and are typically benign.

Now, let’s evaluate the given options:

1. 20% of such tumors are bilateral.

This statement is incorrect. Thecoma-fibromas are usually unilateral. Bilaterality is not common in this type of stromal tumor.

1. Endometrial hyperplasia is often associated.

This statement is correct. Thecoma-fibromas can produce estrogen, which may result in endometrial hyperplasia or even endometrial carcinoma due to the hormone influence.

1. It is always associated with gross ascites.

This statement is incorrect. While ascites can be found in some cases of ovarian tumors, gross ascites is not always associated with thecoma-fibroma. It is more typical of malignant tumors like ovarian carcinoma.

1. It is derived from germ cells.

This statement is incorrect. Thecoma-fibroma tumors derive from stromal cells, not germ cells. Germ cells give rise to different types of tumors, like teratomas or dysgerminomas.

Conclusion:
Based on the above explanations, the most appropriate statement that is true for thecoma-fibroma is "Endometrial hyperplasia is often associated."