Diastereomers are stereoisomers that are not mirror images of each other and have different physical and chemical properties. In the case of amino acids, the presence of a chiral carbon (except for glycine) allows for the existence of stereoisomers.

a) Isoleucine:
Isoleucine is an amino acid with four different substituents attached to its chiral carbon. Therefore, it can exist as two stereoisomers: L-isoleucine and D-isoleucine. These two stereoisomers are non-superimposable mirror images and are therefore enantiomers. Since they are not mirror images of each other, they are also diastereomers.

b) Threonine:
Threonine is another amino acid that has a chiral carbon. It can exist as four stereoisomers: L-threonine, D-threonine, L-allo-threonine, and D-allo-threonine. L-threonine and D-threonine are enantiomers, while L-allo-threonine and D-allo-threonine are also enantiomers. However, L-threonine and L-allo-threonine, as well as D-threonine and D-allo-threonine, are diastereomers since they are not mirror images of each other.

c) Both (a) and (b):
Both isoleucine and threonine have chiral carbons, allowing for the existence of stereoisomers. Isoleucine has two stereoisomers (L-isoleucine and D-isoleucine), which are enantiomers and diastereomers. Threonine has four stereoisomers (L-threonine, D-threonine, L-allo-threonine, and D-allo-threonine), with L-threonine and L-allo-threonine, as well as D-threonine and D-allo-threonine, being diastereomers.

d) None:
Since both isoleucine and threonine have chiral carbons and can exist as stereoisomers, the correct answer is option 'C' - both (a) and (b).