Cucurbita stem is an exceptional dicot stem because it has
A.Bicollateral Bundles
B.Bicollateral bundles and several layered thick pericycles
C.Bicollateral bundles and hollow Centre
D.Bicollateral bundles arranged in two alternate rings

Hint: The arrangement of the vascular arrangement is different as compared to other dicot stems. Two phloem patches, one xylem patch along a pair of cambium forms the vascular bundle in the Cucurbita stem.

Complete answer:
1.Normally in the dicot stems, the phloem made of sclerenchymatous cells occurs as semilunar patches and the xylem lies on the inner side of the dicot stem and the medullary rays passing through the vascular bundles are made up of parenchymatous cell.
2.But Cucurbita stem shows an exception because the stem of Cucurbita shows two phylum patches, one xylem patch, and two cambium which lies in the same circular line. This arrangement of vascular bundles is known as the collateral arrangement.
3.The collateral vascular bundle has phloem on both ends, the outer and the inner phloem where the outer phloem is bordered by the outer cambium strip and the inner phloem is bordered by the inner cambium and the single xylem patch lies between the layer of outer and the inner cambium.

Option D: The Cucurbita stem shows collateral bundles and the vascular bundles are arranged in form of two alternate rings Therefore, this is the correct option.

All other options are irrelevant.

Hence, the correct answer is option (D)

Note: Dicot stems are different from monocot stems in many features like the hypodermis in the dicot stem is collenchymatous whereas in monocot stems the hypodermis is sclerenchymatous. Also, the vascular bundles are arranged in rings in the dicot stem whereas the vascular bundles are scattered in case of monocot stem.