Final answer:
Bismuth has one naturally occurring isotope, ^209Bi, with a 100% natural abundance. Over the lifetime of the universe, only a negligible fraction of bismuth-209 has decayed due to its extremely long half-life.
Step-by-step explanation:
The naturally occurring isotopes of bismuth and their respective percent abundances can be addressed based on scientific data regarding isotopic composition. Bismuth has one naturally occurring isotope, bismuth-209 (^209Bi), which is also considered to be the heaviest stable nuclide. Given this information, the correct answer to the question is: a) ^209Bi (100%), ^208Bi (0%). This means that all natural bismuth is bismuth-209 and there is no natural abundance of bismuth-208.
As for the second part of the question related to the half-life of bismuth-209 and its decay over the lifetime of the universe, it's important to note that bismuth-209 is estimated to have a half-life of 1.9 × 1019 years. The universe is estimated to have a lifetime of about 1.38 × 1010 years, which is much less than the half-life of this isotope. Therefore, over the lifetime of the universe, a negligible fraction of bismuth-209 has decayed, and the percent of bismuth that has decayed would require the use of very small decimal places.