To answer this question, the user needs to have a basic understanding of the OSI (Open Systems Interconnection) model and its different layers.

Bit synchronization refers to the process of coordinating the start and end points of a data transmission between two devices. This ensures that the receiving device can properly interpret the transmitted data.

Now, let's consider each option and see which layer is responsible for bit synchronization:

A. Physical: This layer is responsible for the transmission and reception of raw bit streams over a physical medium. It deals with physical connections and electrical signals. Bit synchronization is indeed handled at this layer, so option A is a correct answer.

B. Network: This layer is responsible for the routing and forwarding of data packets between different networks. It does not deal with the physical transmission of data, so option B is incorrect.

C. Physical: This is just a repeat of option A, so it is also correct.

D. Data Link: This layer is responsible for the establishment and maintenance of reliable data links between adjacent nodes. While it does handle some aspects of bit-level synchronization, such as framing, it is not responsible for the overall synchronization of data transmission. Therefore, option D is incorrect.

E. Presentation: This layer is responsible for the translation, compression, and encryption of data. It has no role in the synchronization of data transmission, so option E is incorrect.

Therefore, the correct answer is:

The Answer is: A. physical