Splenius muscle functions as follows:
A) Together: Flex the neck; Separate: Laterally flex the neck to the same side.
B) Together: Extend the neck; Separate: Rotate the neck to the opposite side.
C) Together: Rotate the neck to the same side; Separate: Extend the neck.
D) Together: Laterally flex the neck to the opposite side; Separate: Flex the neck.