Collateral open vascular bundles and eustele occur in
(A) Monocot stem
(B) Dicot stem
(C) Monocot root
(D) Dicot root

Hint: Collateral open vascular bundles and eustele occur in the plant whose leaves have reticulate venation and have two cotyledons. The part of the plant which has open vascular bundles and eustele shows negative geotropism.

Complete step by step answer:
The stem of dicotyledons has collateral open vascular bundles and eusteles. Eustele is a type of stele found in dicotyledons. In a plant, the stele is present which is defined as the central region of the root or stem containing the tissues derived from the procambium. Collateral vascular strand s is a conjoint vascular bundle where xylem is arranged towards the inner side and thus the phloem is arranged towards the surface. The vascular bundle which has a layer of cambium present between xylem and phloem is known as open vascular bundles.
So, the correct answer is ‘Dicot stem’.

Additional Information: The vascular bundles in monocots are of closed type. Closed vascular bundle means cambium is absent between phloem and xylem. If the cambium is not present, no secondary growth will take place in that type of plant. The stele is of two types i.e Eustele and Atactostele. The major difference between eustele and atactostele is that eustele contains vascular bundles in one or two rings. But, the atactostele contains numerous vascular bundles, which are scattered throughout the stem. Furthermore, eustele occurs in the stem of dicotyledons and the root of monocots, but atactostele occurs in the stem of monocots.

Note: There are four main types of vascular bundles present. They are as follows: Collateral Bundle, Bicollateral Bundle, Concentric Bundle, and at last Radial Vascular Bundle.
- The phloems are observed to be present around the xylem strands in the case of the Collateral bundle.
- In Bicollateral Bundle, the phloem is situated on the peripheral and at the inner side of the xylem.
- In a Concentric Bundle, one type of vascular tissue surrounds the other.
- In Radial Vascular Bundle- In this variety of bundles, the primary vascular tissue i.e the first xylem and first phloem are separated from one another by nonvascular tissues, and that they are situated on alternate radii of an axis.