What term describes the practice of a broker encouraging excessive trading to collect commission fees?

The correct term for the practice of a broker encouraging excessive trading primarily to generate commission fees is churning. Churning occurs when a broker makes unnecessary trades in a client's account to create more activity and, consequently, earn higher commissions. This practice is unethical as it does not align with the best interests of the client, often leading to high fees and diminished returns on investments.

In financial markets, brokers have a fiduciary duty to act in their clients' best interests, and churning violates that principle by prioritizing personal gain over the client's financial prosperity. The other terms mentioned do not encompass this specific behavior related to brokers and trading. Skimming usually refers to taking a portion of profits in an illicit manner, picketing involves demonstration strategies typically unrelated to financial transactions, and lockout pertains to labor disputes and does not relate to brokerage practices. Thus, churning accurately captures the essence of what's described in the question.