What organisms save water by turning carbon dioxide into a special carbon compound before carrying out photosynthesis?

The correct answer is C4 plants.  

A plant that cycles carbon dioxide into four-carbon sugar components to get into the Calvin cycle is known as a C4 plant. These plants are very effective in dry, hot climates and produce ample energy. Various foods one consumes are C4 plants like pineapple, corn, and sugar cane.

In the C4 plants, at the start carbon fixation occurs in the mesophyll cells and the Calvin cycle occurs in the bundle-sheath cells. The PEP carboxylase combines with an incoming molecule of carbon dioxide to the three carbon-molecule PEP, generating a four carbon molecule, that is, oxaloacetate.  

d.embryo is the correct one