Scientists often look at how much isotopes of have decayed to help estimate the age
of a fossil or once-living material.

Scientists often look at how much of isotopes of : Carbon-14  have decayed

To determine the age of fossils archeologist make use of the carbon-14 dating ( i.e. making use of naturally occurring radioisotope ¹⁴C ) which is used to determine the age of fossils or material that were once in living on earth.

Carbon has about three (3) isotopes ( Carbon-12, Carbon-13 and Carbon-14 ) .but Carbon-14 isotope is most useful in determining the age of fossils and once-living things

Hence we can conclude that scientists often look at how much of isotopes of Carbon-14 have decayed to help estimate the age of fossils.

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If im correct it’s radioisotope

Explanation: