My answer assuming you are doing everything right:
Tell him.
PPC
Natural
SEO
Article Marketing
Email Marketing
Banner Advertising
These answers are all legit and general enough to not give away your competitive advantage.
As an AM, I asked plenty of affiliates about traffic sources. Once you give a general answer, the AM will know how to verify whether you are telling the truth or not.
99.9% of my best affiliates would tell the truth, I would verify and our relationship would continue profitably.
The other 0.1% were frauds.
My answer assuming you are doing something shifty:
Dude, you are now *****ed. Suck it up cos you are a scourge on our industry and you deserve not to be paid. In fact I hope you lose money.