Introduction:
A phylloclade is a modified stem structure found in certain plants that resembles a leaf. It is flattened, photosynthetic, and performs the function of a leaf. The term "phylloclade" is derived from the Greek words "phyllon" meaning leaf and "klados" meaning branch. Several plants have evolved this adaptation to overcome water scarcity or to survive in arid conditions. In this question, we are asked to identify which of the given options is not a phylloclade.

Explanation:
Let's examine each option and determine whether it is a phylloclade or not:

1. Opuntia:
Opuntia, commonly known as prickly pear cactus, is a plant genus that belongs to the family Cactaceae. It exhibits phylloclades. The stems of Opuntia are modified into flattened, pad-like structures that resemble leaves. These structures are green, photosynthetic, and perform the functions of leaves.

2. Euphorbia:
Euphorbia is a large genus of plants that belongs to the family Euphorbiaceae. Some Euphorbia species have modified stems called phylloclades. These phylloclades are green, leaf-like structures that are flattened and perform photosynthesis.

3. Ruscus:
Ruscus, commonly known as butcher's broom, is a genus of evergreen shrubs that belongs to the family Asparagaceae. It has modified stems that resemble leaves, called cladodes. Cladodes are flattened, green structures that perform photosynthesis. Therefore, Ruscus does exhibit phylloclades.

4. Asparagus:
Asparagus is a genus of flowering plants that belongs to the family Asparagaceae. It does not have phylloclades. Asparagus has true leaves that are present on its stems. These leaves are small, scale-like, and non-photosynthetic. Therefore, Asparagus does not exhibit phylloclades.

Conclusion:
Among the given options, Asparagus (option D) is not a phylloclade. Asparagus has true leaves, whereas the other options (Opuntia, Euphorbia, and Ruscus) have modified stem structures that resemble leaves and perform photosynthesis.

Asparagus not a phylloclade.