They are characteristics of Dinoflagellates I.e, of the order Pyrrophyta,since,dinoflagellates have stiff cellulose plates which act like protective covering and therefore also known as Armoured algae.Also,most of them have 2 flagella.

Armoured cell wall and biflagellated cells are characteristic of Pyrrophyta.

Explanation:
The given characteristics, armoured cell wall and biflagellated cells, are specific to the group Pyrrophyta, also known as dinoflagellates.

Armoured Cell Wall:
- Pyrrophyta are unicellular organisms that possess a unique cell wall structure.
- The cell wall of dinoflagellates is composed of cellulose plates that are arranged in a specific pattern.
- These plates give the cell wall a rigid and armored appearance, providing protection to the cell.

Biflagellated Cells:
- Pyrrophyta have two flagella, which are whip-like structures used for locomotion.
- These flagella are attached to the cell and are usually located in grooves called sulci.
- One flagellum extends longitudinally, while the other wraps around the cell in a transverse groove.
- The presence of two flagella helps in the movement of dinoflagellates through the water.

Other Characteristics of Pyrrophyta:
- Pyrrophyta are mainly found in marine environments, although some species also inhabit freshwater.
- They are photosynthetic organisms and contain chlorophyll a and c, as well as accessory pigments such as carotenoids.
- Some dinoflagellates exhibit a unique phenomenon known as bioluminescence, where they emit light when disturbed.
- Pyrrophyta play a vital role in marine ecosystems as primary producers and as a food source for other organisms.
- Some species of dinoflagellates are responsible for harmful algal blooms, which can have detrimental effects on marine life and human health.

Therefore, based on the given characteristics of an armoured cell wall and biflagellated cells, the correct answer is option D) Pyrrophyta.