Some protecting groups can protect two hydroxy groups on a carbohydrate at the same time. One such group protects the OH groups at carbon 4 and carbon 6 of β-D-mannose, as shown below. 

a. To what functional group does this protecting group belong? 

b. What reagent is employed in the synthesis of such protected compounds? 

c. When this blocking group is coupled with β-D-mannose, a new chiral center is formed. What is its location? 

d. Draw the compound's other stereoisomer and assess which stereoisomer should be more stable.