What ion is required to maintain the proper integrin structure and to facilitate ligand binding?

calcium

Explanation:

Integrins are principal receptors that bind to the extracellular matrix. They act as transmembrane linkers to link extracellular matrix and the actin cytoskeleton

The adhesive activity of integrins is regulated through two ways –  

a)Within the cell  

b)Out side the cell by extracellular divalent cations  i.e Ca2+ or Mg2+. This shows that there is divalent-cation-binding domains in the extracellular part of the α and β subunits which affects both affinity and the specificity of the ligands to which the integrin binds

i do not know and i have a 100 in this class

it would be c. xylem.