Typically, the inner membrane of mitochondria is highly convoluted to form a series of infolding known as

The correct option is cristae.

To understand why this is the correct answer, let us break down the structure of a mitochondrion and examine the terminology of cellular organelles:

1. Mitochondrial Structure:
Mitochondria are double-membrane-bound organelles found in most eukaryotic organisms. They consist of an outer membrane and an inner membrane. The outer membrane is smooth and acts as a boundary, while the inner membrane is highly folded and convoluted.

2. The Function of Infoldings (Cristae):
These invaginations or inward folds of the inner mitochondrial membrane are specifically called cristae (singular: crista). The primary purpose of these folds is to greatly increase the surface area of the inner membrane. This expanded surface area accommodates a larger number of electron transport chain proteins and ATP synthase enzymes, thereby maximizing the cell's capacity to generate chemical energy in the form of ATP during cellular respiration.

3. Why the other options are incorrect:
- Grana: These are stacks of disc-shaped thylakoids found inside chloroplasts (the sites of photosynthesis in plant cells), not mitochondria.
- Thylakoids: These are membrane-bound compartments inside chloroplasts where the light-dependent reactions of photosynthesis take place.
- Lamellae: These are thin, plate-like membrane structures that connect grana inside chloroplasts (stroma lamellae), or refer to structural layers in other biological contexts, but they do not define the convoluted inner folds of mitochondria.