The correct option is
B calculating the ratio of C-14 to C-12 in the fossils
Fossils are the preserved remains, traces or impressions of organisms or their parts that lived in a past geological age. These could include the whole body of the extinct organism, inorganic remains of plants and animals, remains of bones or even moulds, casts and footprints embedded in stratified rocks.
To find out the approximate age of the fossils, a carbon dating method is commonly used. This is based on the principle of radioactive decay. All living organisms consume different amounts of isotopes of carbon (C-12 and C-14). C-12 is stable but C-14 is unstable and is radioactive.
When a plant or animal dies, the consumption of carbon stops but the decay of C-14 continues at a slower rate (it takes around 5730 years for 1g of radioactive C-14 to become 0.5g) until all of it has decayed. The disintegration of C-14 into C-12 remains constant. Hence, the quantity of C-14 relative to C-12 varies progressively with decay. Assuming that the decay rate is not affected by the prevailing environment, the relative age in which the organism existed can be determined by this method. The older the fossil is, the lesser the quantity of C-14 present in the sample.
Measuring the depth of excavated fossils also determines the age of fossils. But it does not involve the measurement of radioactive decay of carbon, which is the heart of the carbon dating process.
Comparing the size of organs that are vestigial in present-day species with the fossils cannot help us to determine the age of fossils.