The correct option is A think
The actions 'to think' and 'to know' must agree in terms of tense, as this sentence refers to a fact, from the speaker's point of view.
Option B: The tense form 'I know' is in simple present; it describes a state or fact. Hence the underlined verb must also be in the same tense form, which is 'I think'. So, option B is correct.
Option A: 'Thinks' is also in the simple present tense but it is used when the subject is in the third person narrative 'he/she thinks'. But, the subject 'I' is in the first person, hence A is incorrect.
Option C: 'Was thinking' is in the past continuous tense whereas the sentence is in present tense. Hence C is incorrect.
Option D: 'Have been thinking' is in the present perfect continuous tense whereas the sentence is in simple present tense. Hence D is incorrect.