He has always been an elusive customer.

When we say someone is elusive, it means they are difficult to find, catch, or achieve.

An elusive customer is someone who is hard to pin down, difficult to contact, or inconsistent in their presence or commitment.

Option B: unreliable is the best synonym because it refers to someone who cannot be depended upon.

Such a customer may not show up when expected or behave inconsistently, which closely matches the idea of being elusive.

Option A: ambiguous means unclear or having multiple meanings, which does not fit this context.

Option C: deceitful means dishonest, which is different from simply being hard to reach.

Option D: quarrelsome means argumentative, which is unrelated to the meaning of elusive.

Therefore, the closest meaning to elusive here is unreliable.