To answer this question, we need to understand the correct usage of prepositions in English.
The correct option is A) "For."
The preposition "for" is used to indicate a duration of time. In this case, the sentence is referring to the duration of time that John and Bob have been working on the project, which is five months. Therefore, the correct sentence should be "John and Bob have been working on this project for five months already."
Option B) "After" is incorrect because it is used to indicate a sequence of events, not a duration of time. For example, "After finishing the project, they celebrated."
Option C) "Since" is incorrect because it is used to indicate a starting point in time. For example, "They have been working on this project since January."
Option D) "None of these" is incorrect because option A is the correct answer.
Therefore, the correct option is A) "For."