The most noticeable organelle in a eukaryotic cell under a microscope is the nucleus.

Here's why:

* Size: The nucleus is typically the largest organelle in the cell.

* Shape: Its spherical shape stands out against other organelles.

* Staining: The nucleus stains readily with dyes, making it easier to distinguish from other structures.

While other organelles like mitochondria and chloroplasts are also visible under a microscope, their smaller size and less distinct staining often make them less noticeable than the nucleus.