Which of following compound has center of symmetry?

Meso-tartaric acid is

Which will have enantiomer

The number of optical isomers in following compound is:. . . . . . 

Allegra, a common prescription drug with the structure shown below, is given for  the treatment of seasonal allergies. How many stereogenic carbon does Allegra possess ?

Determine the absolute configurations of the labeled carbons ($a$ and $b$)