The correct option is
A Me
In order to fill in the blank with the correct form of the pronoun, let's understand what objective case (accusative case) is. An accusative case is an objective case. These pronouns are objects in a sentence. For example, 'He said that he misses them.' Here, 'them' is the object and hence it is an accusative case of a pronoun.NOTE: As the sentence is in first person form, we need the objective pronoun to be in the first-person form as well.
Option A - 'I' is a first-person singular pronoun, but it a subjective case of the pronoun. As we need to fill in the blank with an objective case of the pronoun, we can't use 'I', Hence, option A is incorrect.
Option C - 'We' is a first-person plural pronoun and as we need a singular pronoun, we can't use 'we'. Hence, option C is incorrect.
Option D - 'They' is a third-person plural pronoun, Hence, option D is incorrect.
Option B - 'Me' is a first-person singular objective pronoun. Hence, we can use it here. Thus option B is the correct answer.
'This is a secret pact between you and me.'