Any organic material from an organism.
Carbon-14 is only one form of carbon; others have differing mass numbers, e.g. carbon-12, carbon-15. Thus it fits the definition of an isotope.
Isotopes are chemically same but physically differ, carbon-12 and carbon-14 have different numbers of neutrons so have different mass numbers.
Only once living things (plant or animal). The measurements depend on an even mixture of the isotopes, which respiration takes care of.
(1) After several times the half-life, the amount of C-14 that remains is so small that it is hard to measure accurately. (2) It is only useful when you can assume that a certain number of years ago,...