Hypodermis is the correct answer.

The monocot stem is a type of stem found in monocotyledonous plants, which are characterized by having a single cotyledon in their seed. The stem is composed of different layers, each with its own specific functions. The layer just below the epidermis in a monocot stem is called the hypodermis.

The hypodermis is a layer of cells located just below the epidermis. It is made up of parenchyma cells, which are loosely packed and have thin cell walls. The hypodermis is mainly responsible for providing mechanical support to the stem. The parenchyma cells in this layer have the ability to elongate, which helps in maintaining the shape and structure of the stem.

The hypodermis also functions as a storage tissue. It stores nutrients and water, which can be used by the plant during periods of drought or nutrient deficiency. Additionally, the hypodermis helps in the conduction of water and nutrients from the roots to the rest of the plant. It contains specialized cells called transfer cells, which have numerous finger-like projections that increase the surface area for efficient nutrient and water absorption.

In contrast, the endodermis is a layer of cells found in the roots of plants. It is responsible for controlling the movement of water and minerals into the vascular tissues of the root. The vascular bundles, on the other hand, are specialized tissues that are responsible for the transport of water, nutrients, and other substances throughout the plant.

In summary, the hypodermis is the layer just below the epidermis in a monocot stem. It provides mechanical support, stores nutrients and water, and helps in the conduction of water and nutrients.