I have been thinking about this _______ yesterday.

The correct answer is 'since' because the sentence structure requires a preposition that links the present perfect continuous tense ('have been thinking') with a specific starting point in time. The word 'yesterday' indicates when the action began, and 'since' is specifically used to denote a starting point in time that continues up to the present moment. For example, 'I have been thinking about this since yesterday' correctly implies that the thinking started in the past and is still ongoing.\nOption A ('along') and Option B ('by') are incorrect because they do not function as time markers in this context; 'along' suggests movement or accompaniment, while 'by' typically indicates proximity or agency. Option C ('after') is also incorrect because it refers to a time following an event, which would not logically connect with the present perfect continuous tense indicating ongoing action from a past point. Therefore, 'since' is the only grammatically appropriate choice to complete the sentence.