The correct answer is (a) 12.75 million.
The total population is 100%. The percentage of people below 36 years of age is 30%. Therefore, the percentage of people above 36 years of age is 70%. The percentage of people in the age group 56-65 is 5.12%. Therefore, the number of people in the age group 56-65 is 5.12% of the total population, which is 5.12% * 100 million = 5.12 million. Since the total population is rounded to the nearest million, the number of people in the age group 56-65 is also rounded to the nearest million, which is 12.75 million.
Option (b) is incorrect because it is the percentage of people in the age group 36-45. Option (c) is incorrect because it is the percentage of people in the age group 46-55. Option (d) is incorrect because it is not a specific number.