An action in a sentence can be represented in two ways, namely active and passive voice.

The passive voice always uses the past participle form of the main verb irrespective of any tense. Only the auxiliary verbs depend upon the sentence given in the active voice.

The option B is correct as the given sentence is in the active voice and to transform it into passive voice the subject of the given sentence 
(we) comes in the place of the object (the end of this exercise) and vice-versa.

Also, as the given sentence is in the present continuous tense, 'is being' is used before the past participle form of the main verb (end). 

Options A, C, and D are wrong as the subject is 'the end of this exercise', not 'this exercise', the object of the given sentence does not come in the place of the subject of the given sentence, and the sentence is not in present perfect tense respectively.

The correct answer is B)The end of this exercise is being reached by us.