The average atomic mass recorded on the periodic table for krypton is 83.80 u. this indicates that the most abundant isotope of krypton is (2 points) krypton-82 krypton-83 krypton-84 krypton-85

Answer is: krypton-84.

Isotopes are chemical elements with same atomic number, but different atomic mass (different number of neutrons).
The average atomic mass of krypton = 
= ω(⁸²Kr) · Ar(⁸²Kr) + ω(⁸³Kr)·Ar(⁸³Kr) + ω(⁸⁴Kr) · Ar(⁸⁴Kr) + ω(⁸⁵Kr)· Ar(⁸⁵Kr).
ω(Kr) is average abundance of isotopes of krypton.
Ar(Kr) is atomic mass of krypton isotope.
Average atomic mass of krypton is closest to Kr-84.

The correct answer is "hydrogen".