The common feature between the flowers of spadix,cyathium and hypanthodium are declinous flowers.It is the condition in which the plants have stamens and pistils on separate flowers, it could be on the same plant or different plants.

The correct answer is option 'C' - Declinous.

Explanation:
To understand why the correct answer is 'C' - Declinous, let's first define and understand the terms associated with the flower structures mentioned in the question.

1. Spadix: A spadix is a type of inflorescence found in certain flowering plants. It is a spike-like structure with small, densely packed flowers attached directly to a fleshy stem, known as the spadix. Examples of plants with spadix inflorescence include the Araceae family, which includes the commonly known plants like the Peace Lily (Spathiphyllum) and the Philodendron.

2. Cyathium: A cyathium is a specialized type of inflorescence found in plants belonging to the Euphorbiaceae family. It consists of a cup-like structure called the involucre, which encloses multiple small, unisexual flowers. The cyathium inflorescence is characteristic of plants like the Euphorbia genus, which includes plants like the Poinsettia (Euphorbia pulcherrima).

3. Hypanthodium: A hypanthodium is a unique type of inflorescence found in plants belonging to the Moraceae family. It is a hollow, urn-shaped structure formed by an enlarged, fleshy receptacle. Inside the hypanthodium, numerous tiny, unisexual flowers are borne. The common fig (Ficus carica) is an example of a plant with a hypanthodium inflorescence.

Now, let's understand the term 'declinous' and its relevance to the flower structures mentioned above.

- Declinous: Declinous refers to a type of flower that lacks stamens (male reproductive organs) and is therefore functionally male-sterile. In other words, declinous flowers do not produce pollen.

Considering the flower structures mentioned in the question, all three - spadix, cyathium, and hypanthodium - have flowers that lack stamens. Therefore, they are all declinous.

To summarize, the common feature of the flower structures mentioned - spadix, cyathium, and hypanthodium - is that they all have declinous flowers, i.e., flowers that lack stamens. This knowledge is important in understanding the reproductive biology and classification of these plant species.