Federal Reserve Regulation T sets the initial margin requirement for purchasing equity securities at 50%. This means an investor may borrow no more than 50% of the purchase price from the broker-dealer and must deposit at least 50% in equity. Choice C is correct. For example, if a customer purchases $10,000 of marginable stock, the customer must deposit $5,000 and may borrow $5,000. Choice A and choice B understate the amount permitted to be borrowed under the initial margin rule. Choice D is incorrect because customers may not borrow the entire purchase price of an equity security in a margin account. Regulation T is a Federal Reserve Board rule and is a central SIE topic in margin accounts and customer trading. The SIE outline includes cash and margin accounts, margin disclosure, Federal Reserve Board Regulation T, and margin requirements. This question tests the initial margin loan limit, not maintenance margin or house margin. Maintenance requirements are separate and may be imposed by FINRA, exchanges, or the firm. Reference: Understanding Trading, Customer Accounts and Prohibited Activities; Margin Accounts; Federal Reserve Regulation T; FINRA Rule 4210.