Vascular bundles are closed in monocots as:

The correct option/answer is "absence of vascular cambium".

In vascular plants, the vascular bundle is a part of the transport system consisting of xylem (which conducts water and minerals) and phloem (which conducts organic food material). These bundles are classified as "open" or "closed" based on the presence or absence of a meristematic tissue called vascular cambium.

Vascular cambium is a lateral meristem responsible for secondary growth (increase in girth/thickness) in plants.
When cambium is present between the xylem and phloem (as seen in dicotyledonous plants), the vascular bundles are described as open because they retain the capacity to form secondary xylem and phloem tissues.

Conversely, in monocotyledonous plants (monocots), there is no vascular cambium present between the xylem and phloem. Because of this absence of vascular cambium, these plants cannot undergo secondary growth, and their vascular bundles are described as closed.