The correct answer is d) ratio.
Age, when measured in years, is considered a ratio variable because it possesses all the properties of an interval variable, along with the added feature of a true zero point. In this context, a zero age signifies an absence of age, which means that it is meaningful to say that someone who is 40 years old is twice as old as someone who is 20. This ratio aspect allows for meaningful comparisons between ages, making it distinctly a ratio scale.