Pandas: Assign column from single-indexed df to multi-indexed df

Question

Consider three dataframes:

df1 = pd.DataFrame({\'x1\': [1,2,3], \'a\': [1,1,2], \'b\': [10,20,10], \'c\':[100, 200, 300]}).set_index([\'a\', \'b\', \'c\'])
df