(a) Index the items of a using the elements of b:

>>> [a[x] for x in b]
['E', 'C', 'G', 'B', 'F', 'A', 'D']

(b) Index the items of a using the sorted elements of b. In this case, the returned list is just (a copy of) a:

>>> [a[x] for x in sorted(b)]
['A', 'B', 'C', 'D', 'E', 'F', 'G']

(c) Index the items of a using the elements of b indexed at the elements of b(!)

>>> [a[b[x]] for x in b]
['F', 'G', 'D', 'C', 'A', 'E', 'B']

(d) Associate each element of b with the corresponding element of a in a sequence of tuples: [(4, 'A'), (2, 'B'), (6, 'C'), ...] which is then sorted — this method is used to return the elements of a corresponding to the ordered elements of b.

>>> [x for (y,x) in sorted(zip(b,a))]
['F', 'D', 'B', 'G', 'A', 'E', 'C']