The correct answer is C. 6.
A double sided magnetic disk pack has six disks. Each disk has two surfaces, so the total number of surfaces is 6 x 2 = 12. However, only one surface of each disk is normally used, so the total number of surfaces that are normally used is 12 / 2 = 6.
Option A is incorrect because it is the total number of surfaces in the disk pack, not the number of surfaces that are normally used.
Option B is incorrect because it is the total number of surfaces in the disk pack plus one. This is because each disk has two surfaces, so the total number of surfaces in the disk pack is 6 x 2 = 12. However, only one surface of each disk is normally used, so the total number of surfaces that are normally used is 12 / 2 = 6. Adding one to this number gives 6 + 1 = 7, which is not an option.
Option C is the correct answer because it is the number of surfaces that are normally used.
Option D is incorrect because it is the total number of surfaces in the disk pack minus one. This is because each disk has two surfaces, so the total number of surfaces in the disk pack is 6 x 2 = 12. However, only one surface of each disk is normally used, so the total number of surfaces that are normally used is 12 / 2 = 6. Subtracting one from this number gives 6 – 1 = 5, which is not an option.
Option E is incorrect because it is not one of the possible answers.