Folding of the inner membrane of mitochondria are called

What is the energy currency of the cell?

The fluid-filled matrix present inside the mitochondria is called .......

The matrix of the mitochondrion contains .......

Assertion $A$: Mitochondria are the powerhouse of the cell.
Reason $R$: The sites for cellular respiration,specifically the $Krebs$ cycle and oxidative phosphorylation,are located within the mitochondria.
Which option is correct for Assertion $A$ and Reason $R$?

Which of the following organelles contains a system of folded membranes?