To understand why the correct answer is option 'B', let's first define what the notation A - B means.

A - B represents the set of elements that are in set A but not in set B. In other words, it is the set of elements that are in A and not in B.

Explanation:
Let's suppose A = {1, 2, 3, 4} and B = {3, 4, 5, 6}. Now let's calculate A - B.

- We start with set A = {1, 2, 3, 4}.
- We remove all the elements from A that are also in set B, which are 3 and 4.
- Therefore, A - B = {1, 2}.

Now let's consider the options given:

a) AB: This option represents the intersection of sets A and B, which is the set of elements that are common to both A and B. This is not the same as A - B, so it is not the correct answer.

b) AB: This option represents the set of elements that are in A but not in B, which is exactly what A - B represents. Therefore, this is the correct answer.

c) AB: This option represents the union of sets A and B, which is the set of all elements that are in either A or B. This is also not the same as A - B, so it is not the correct answer.

d) None of these: This option implies that none of the given options is correct. However, option 'B' represents A - B, which is the correct answer.

Therefore, the correct answer is option 'B' (AB), as it represents the set of elements that are in A but not in B.