Salutations!
Which of the following terms best describes the Islamic religion?
Ummah means community, a community of all Muslims, so option A is false. Polytheist are people who worship more than one God, so option B is false again. Shariah means the religious laws followed by the Muslim people, so option D is false too. Thus, your answer is option B, which is monotheist. Monotheist are people who worship only one God. All Muslim people are monotheists.
Hope I helped!