.............. technique is used to know the sex of developing foetus.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks for the technique used to determine the sex of a developing fetus.

2. **Identifying the Technique**: The technique that is commonly used for this purpose is called "amniocentesis."

3. **What is Amniocentesis?**: Amniocentesis is a medical procedure where a small amount of amniotic fluid is extracted from the amniotic sac surrounding the developing fetus.

...