Question about reset_index

[XXXVivilovesBoba]

Hi Emma,
For Exercise 6. Find the top 3 universities with the largest numbers of undergraduate students for each state.
My code is listed below:
state2 = df[['stabbr','ugds','instnm']]
state3 = state2.groupby(["stabbr"]).apply(lambda x: x.sort_values(["ugds"], ascending = False)).reset_index(drop=True)
df4 = state3.groupby('stabbr').head(3).head(100)

I do not understand why I have to reset_index for this question.

And another question,
for the code listed below:
result = df[['stabbr','ugds','instnm']]
result.groupby('stabbr').apply({'ugds':{lambda x: x.nlargest(3)}})
I got the error message: TypeError: unhashable type: 'dict'
I do not understand why.

I appreciate your help.