For this case you should be aware of the correct conjugation of the verb to be in the present simple.
The conjugation in English is given by:
I am
you are
he/she/it is
we are
you are
they are
Therefore, the sentence written correctly in English is given by:
I am not so smart
The sentence in Spanish in subjunctive presents is:
Quizá yo no sea tan inteligente
Answer:
b. sea
Quizá yo no sea tan inteligente