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I am confused with the grammar of comparative degree. Which of the following option is acceptable?

My sisiter is a college student. She is usually a little busier than _________.

(1) me (2) I (3) I am
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In real English, they are all acceptable.

#1 is casual or informal
#2 is formal or hyperformal
#3 is formal or neutral

They expect you to answer with #3. (And the spelling is 'sister'.)
Comments  
How about the following sentences?
Are there any "be-Verb"s?

She is busier than Mike is. or She is busier than Mike.
I'm older than he is. or I'm older than he.
I'm older than my brother is. or I'm older than my brother.

Are they all negligible?