Final answer:
The correct line of code to add a 0.4-second delay to a program is b) Sleep (400).
Step-by-step explanation:
The correct line of code to add a small delay of 0.4 seconds to a program is Sleep (400). This line of code tells the program to pause execution for 400 milliseconds, which is equivalent to 0.4 seconds. The other lines of code, such as Sleep (500), Sleep (3000), and Sleep (5000), represent longer delays.