Answer: The age of a rock in years is known as D. absolute age.
Explanation: When geologists and scientists date a fossil or a rock using geologic methods, such as by measuring the amount of radioactive material in it, they are calculating the "absolute age" of that rock/fossil.
Index age is a term probably invented by the one who made this question, and it does not exist. Relative age might have been a tempting choice, but it is not really correct as it is like "looking at a cake." Scientists estimate the age of a layer by the order of where it is. Half life age is used to calculate the time required for a substance to be reduced half of its original value.