I think that in the end the crux is what it means by "practice"
In everything there is a 'theory' and then there is the 'practice' of that theory.
The 'theory' of transfer pricing is to redirect profits to take advantage of not neccesarily 'tax havens', but countries with less tax, yes.
The 'practice' of this - or HOW this theory is eventuated - is via option (b), although again, assumptions must be made as to who is the high tax group and who is the low tax group.
(a) does not really 'describe the practice' but rather the outcome OF the 'practice'. That is to say, by engaging in transfer pricing, profits are redirected, but profits being redirected is not the 'practice' of transfer pricing.
In sum, look at this question in terms of a 'means' to an 'end' question. The 'practice' of something implies the 'means' while the ideology/theory behind why one would implement that 'practice' is the 'ends'.
(a) is the 'end' or the ideology/theory
(b) is the 'means' or the 'practice'
Thus (b) is MOST correct answer imho