Selectivity in acetonide formation from a polyol
Acetonide formation (isoproylidine ketal formation, to be more correct) is indeed a thermodynamic process, and under acidic conditions acetonides will tend to migrate to form the lowest energy product. 

If you look at the penta-ol you've been given, there are three possible permutations containing two acetonides (theres also the possibility of just forming a single acetonide, but these kinds of reactions are usually ran with an excess of the acetone/dimethoxypropane, so its likely to go on as many times as possible):  



The green one (which is the one labelled A in your image) is the thermodynamic product, which becomes obvious if you consider the three-dimensional conformation of 1,3-syn and 1,3-anti acetonides (note that this analysis only holds up then there is no substituent at the 2-position. If there is a methyl group there, as is common in polyketide type motifs, then the acetonides don't necessarily adopt chair conformations):

 

In the syn acetonides, there are three equatorial and one axial groups (minimal strain), whilst in the anti acetonides (which both of the un-observed products contain one of), there are two equatorial and two axial groups, giving a 1,3-diaxial clash across the ring (hence raising the energy of these products).