The correct answer is 3 - This describes the symmetrical tonic neck reflex (STNR). The STNR is triggered by changes in the infant's head and neck position. Specifically, it occurs when the infant flexes or extends their neck:
When the infant's head is flexed, the arms flex at the elbows while the legs extend at the knees.
When the infant's head is extended, the arms extend at the elbows while the legs flex at the knees.