Extensive infoldings of mitochondrial membrane are ..........

Explanation

**Step-by-Step Solution:**

1. **Understanding the Structure of Mitochondria**: Mitochondria are double-membraned organelles found in eukaryotic cells. They consist of an outer membrane and an inner membrane.

2. **Identifying the Inner Membrane**: The inner membrane of the mitochondria is where we find extensive infoldings. These infoldings are crucial for the function of the mitochondria.

3. **Naming the Infoldings**: The extensive infoldings of the inner mitochondrial membrane are called "cristae." These structures increase the surface area of the inner membrane.

...