The correct option is D It is used to describe habits.
The tense of a sentence denotes a time, whether it is the time now (present), time then (past) or the time to come (future). The verb of a sentence has most to do with the tense. A sentence in the Simple Present Tense describes a general truth, or an action that takes place regularly, like a habit or routine, I travel by bus to school, where 'travel' is in the simple present tense. It's mentioned in option A that it refers to a habit, hence option A is correct. Option B describes the present continuous tense, which can describe a changing situation, or an ever-changing situation, like in 'global warming is increasing by the minute'. Similarly, the present continuous tense describes temporary plans, as in 'I am working on my phone till my laptop gets repaired'. It does not refer to a routine, or a general truth. These situations are in the continuous form, and not the simple present tense, hence options B, C and D are incorrect.