Explanation:

In plants, the vascular bundles are responsible for the transport of water, minerals, and nutrients throughout the plant. These vascular bundles consist of xylem and phloem tissues. The arrangement and structure of these vascular bundles vary in different plant parts.

Monocot Stem:
In monocot stems, the vascular bundles are scattered throughout the ground tissue. Each vascular bundle is enclosed within a well-developed sclerenchymatous sheath. The sclerenchyma cells provide mechanical support to the vascular bundle and protect it from damage.

Dicot Stem:
In dicot stems, the vascular bundles are arranged in a ring-like pattern called the vascular cambium. The vascular cambium separates the xylem and phloem tissues. Each vascular bundle in dicot stems is not enclosed within a well-developed sclerenchymatous sheath like in monocot stems.

Monocot Root:
In monocot roots, the vascular bundles are also scattered throughout the ground tissue. However, in contrast to monocot stems, the vascular bundles in monocot roots are not enclosed within a well-developed sclerenchymatous sheath. Instead, the vascular bundles are surrounded by parenchyma cells.

Dicot Root:
In dicot roots, the vascular bundles are arranged in a central cylinder-like structure called the stele. The stele is surrounded by the endodermis, which acts as a protective layer for the vascular bundles. The endodermis in dicot roots is made up of specialized cells called the Casparian strip, which helps regulate the movement of water and nutrients.

Conclusion:
Therefore, the correct answer is option 'A' - monocot stem. In monocot stems, the vascular bundles are enclosed within a well-developed sclerenchymatous sheath, providing mechanical support and protection to the vascular bundles.