Here's why:

* Prokaryotic cells are simple, single-celled organisms like bacteria and archaea. They lack a nucleus and other membrane-bound organelles.

* Eukaryotic cells are more complex and found in plants, animals, fungi, and protists. They have a nucleus that contains their DNA, and they have various other membrane-bound organelles like mitochondria, chloroplasts, and the endoplasmic reticulum.

Oak leaves are part of a complex multicellular organism (a tree) and therefore their cells are eukaryotic.