(b) 1600
In a 4-for-1 stock split, each existing share becomes 4 shares. So, 400 × 4 = 1600 shares.
The source passage directly uses this exact example: an investor holding 400 shares after a 4-for-1 split holds 1600 shares. Multiply existing shares by the split ratio (4). Total value remains unchanged as the share price simultaneously reduces to 1/4th.