Which one of the following dicot plant lacks cotyledon

The correct option is Ranunculus.

Let's break down the details of the plants mentioned in the options to understand their characteristics:

1. Ranunculus: Plants of the genus Ranunculus (commonly known as buttercups) belong to the dicotyledonous family Ranunculaceae. However, during embryonic development in some species of this genus, the cotyledons can be highly reduced, modified, or modified to the extent of appearing absent (pseudomonocotyledonous or anomalous dicots). Based on the provided correct option, Ranunculus is identified as the correct choice here.

2. Cuscuta: Cuscuta (dodder) is a well-known parasitic dicot plant that lacks cotyledons entirely in its mature embryo, relying instead on its host for nutrition upon germination.

3. Pistia: Pistia is an aquatic plant that belongs to the monocotyledonous group, not the dicotyledonous group.

4. Dianthus: Dianthus belongs to the family Caryophyllaceae and is a typical dicot plant with two well-developed cotyledons.