Questions: C4 plants differ from C3 and CAM plants in that C4 plants - open their stomata only at night - use malic acid to transfer carbon dioxide to the Calvin cycle - use PEP carboxylase to fix carbon dioxide - transfer fixed carbon dioxide to cells in which the Calvin cycle occurs - are better adapted to wet conditions

The answer is: use PEP carboxylase to fix carbon dioxide.

Explanation for each option:

1. Open their stomata only at night: This is characteristic of CAM (Crassulacean Acid Metabolism) plants, not C4 plants. CAM plants open their stomata at night to minimize water loss in arid conditions.

2. Use malic acid to transfer carbon dioxide to the Calvin cycle: While C4 plants do use malic acid as an intermediate, this statement is not specific enough to distinguish C4 plants from CAM plants, which also use malic acid in their carbon fixation process.

3. Use PEP carboxylase to fix carbon dioxide: This is correct. C4 plants initially fix carbon dioxide using the enzyme PEP carboxylase, which forms a four-carbon compound (oxaloacetate) that is later converted to malate or aspartate. This process helps to concentrate CO2 in the bundle-sheath cells where the Calvin cycle occurs, reducing photorespiration.

4. Transfer fixed carbon dioxide to cells in which the Calvin cycle occurs: This is also true for C4 plants, but it is not as specific as the use of PEP carboxylase for initial carbon fixation, which is a defining feature of C4 photosynthesis.

5. Are better adapted to wet conditions: C4 plants are generally better adapted to high light, high temperature, and dry conditions rather than wet conditions. This statement is incorrect.

Summary: C4 plants are characterized by their use of PEP carboxylase to fix carbon dioxide, which helps them efficiently carry out photosynthesis in conditions where C3 plants might suffer from photorespiration.