Explanation:
The correct option is 'a'. Atlanto-occipital dissociation (AOD) is a highly fatal craniocervical injury. The Harris lines (Basion-Dental Interval [BDI] and Basion-Axial Interval [BAI]) are the most accurate plain radiographic measurements to detect it. By the Rule of 12s, both the BDI and BAI should normally be less than 12 mm on a lateral radiograph. Values exceeding this indicate craniocervical instability.

Reference: Harris JH Jr, Carson GC, Wagner LK. Radiologic diagnosis of traumatic occipitovertebral dissociation: 1. Normal occipitovertebral relationships on lateral radiographs of supine subjects. AJR Am J Roentgenol. 1994;162(4):881-886.