Answer: lead-207
Step-by-step explanation:
The average atomic mass of an element's atom is all the isotopes of that element added together, then divided by the number of isotopes, to get the average. So, the element's most abundant isotope will be the isotope that is closest to the element's average atomic mass.
Lead's average atomic mass is 207.2 amu, so we look at the isotopes of lead to find the one that has the value closest to 207, which would be lead-207.
I hope this helps! :)