correct answer is b.
Any time an investor uses SMA, he/she is borrowing money from the broker-dealer and increasing the DR balance. SMA has a purchasing power equaling the SMA x 2 ($1,600 x 2 = $3,200). The broker-dealer will loan the client an additional $3,200 to purchase stock. Add this to the existing DR balance of $6,400 + $3,200 = $9,600.