Krypton-84 is the most abundant type of Krypton. The answer is C.
If you need this to be explained, I will do my best it's a bit difficult to say how to find it out, but I will if you need me to.
The electron configuration of Lithium is 1s2 2s1
One mole of water weighs 18 grams. H₂O is composed of 2H= 2 and 1 0=16 adding gives you 18. number of moles= mass/ Relative Molecular Mass
Therefore, mass= Relative Molecular Mass×number of moles
= 18×5 moles
= 90 grams