The principal metabolic feature of CAM plants is assimilation of CO2 at night into malic acid which is stored in the vacuole. Malate is generated in the reaction catalyzed by PEP carboxylase and PEP is, in turn, generated by degradation of starch or soluble sugars. During the day, malate is released from the vacuole and is decarboxylated to provide CO2 for fixation in the Benson–Calvin cycle behind closed stomata. Starch and sugars are then resynthesized .

CAM (Crassulacean Acid Metabolism) plants are a type of plants that have adapted to arid environments by utilizing a unique carbon fixation pathway. In these plants, the fixation of carbon dioxide occurs during the night, and the fixed carbon is stored as organic acids. This process is called ossification.

The correct answer is option C, which states that malic acid is produced during ossification in CAM plants. Let's understand why malic acid is the correct answer.

Ossification in CAM Plants:
1. Carbon Fixation during the Night:
- CAM plants open their stomata at night to minimize water loss through transpiration.
- Carbon dioxide from the atmosphere enters the leaf through open stomata and combines with phosphoenolpyruvate (PEP) to form oxaloacetate.
- This reaction is catalyzed by an enzyme called phosphoenolpyruvate carboxylase (PEPC).

2. Conversion to Malic Acid:
- Oxaloacetate is converted into malate (malic acid) by a series of enzymatic reactions.
- The conversion involves the reduction of oxaloacetate to malate using NADH.
- The enzyme malate dehydrogenase (MDH) catalyzes this reaction.

3. Storage of Malic Acid:
- The malic acid formed during ossification is stored in vacuoles within the plant's cells.
- This storage allows the plant to utilize the stored carbon dioxide during the day when the stomata are closed to prevent water loss.

4. Carbon Release during the Day:
- During the day, when the stomata are closed, the stored malic acid is decarboxylated to release carbon dioxide.
- This carbon dioxide enters the Calvin cycle, a light-independent reaction that occurs in the chloroplasts, to produce glucose and other organic compounds.

5. Benefits of Ossification in CAM Plants:
- By fixing carbon dioxide during the night and storing it as malic acid, CAM plants can minimize water loss through transpiration during the day.
- This adaptation allows them to survive in arid environments with limited water availability.

In conclusion, the dark fixation of carbon dioxide in CAM plants is called ossification because it produces malic acid. Malic acid is formed by the conversion of oxaloacetate, which is the product of carbon dioxide fixation during the night. The stored malic acid is subsequently decarboxylated during the day to release carbon dioxide, which is then used in the Calvin cycle for glucose production.