This is how to conjugate the verb ESTAR (which means to be) in the present tense:
yo estoy, tu estás, el/ella/Usted está, nosotros estamos, vosotros estáis, ellos/ellas/Ustedes están.
Your sentence is I am home. In Spanish, the word YO stands for I in English. This means that you would use the form estoy in your example.
The correct answer is B. estoy.