Consider a data frame df with 10 rows and index [ 'r1', 'r2', 'r3', 'row4', 'row5', 'row6', 'r7', 'r8', 'r9', 'row10']. What does the expression g = df.groupby(df.index.str.len()) do?

Consider a data frame df with 10 rows and index [ ‘r1’, ‘r2’, ‘r3’, ‘row4’, ‘row5’, ‘row6’, ‘r7’, ‘r8’, ‘r9’, ‘row10’]. What does the expression g = df.groupby(df.index.str.len()) do?
a. Groups df based on index values
b. Groups df based on length of each index value
c. Groups df based on index strings
d. Data frames cannot be grouped by index values. Hence it results in Error.