Given,
The two isotopes of B are 10B and 11B
% abundance of 10B = 19.80
% abundance of 11B = 80.20
Average atomic mass of B
= 
= 
= 
= 
= 10.802
Therefore, the average atomic mass of B is 10.802 u
Answer: d. both (b) and (c)
Explanation:
Atoms are too small to be seen on microscopes, so we model them based on theories.