The correct answer is option 'A': network layer.

Explanation:
The transport layer is responsible for providing end-to-end communication between two devices on a network. It ensures that data is delivered reliably, accurately, and in the correct order. One of the key functions of the transport layer is to aggregate data from different applications into a single stream before passing it to the network layer.

Here is a detailed explanation of each layer and their responsibilities:

1. Physical Layer:
- The physical layer deals with the physical transmission of data over the network.
- It defines the physical characteristics of the network such as the type of cables, connectors, and signaling methods.

2. Data Link Layer:
- The data link layer is responsible for the reliable transmission of data between two directly connected devices.
- It breaks the data into frames and performs error detection and correction.
- It also handles flow control and manages access to the physical medium.

3. Network Layer:
- The network layer is responsible for routing packets of data from the source to the destination across multiple networks.
- It adds addressing and routing information to the data packets.
- It determines the best path for data transmission based on network conditions and congestion.
- It deals with logical addressing and provides logical connectivity between hosts.

4. Transport Layer:
- The transport layer provides end-to-end communication between two devices.
- It ensures reliable and accurate delivery of data by adding sequence numbers, acknowledgments, and error-checking.
- It also handles flow control, congestion control, and multiplexing.
- Multiplexing is the process of combining data from different applications into a single stream for transmission over the network.

In summary, the transport layer aggregates data from different applications into a single stream before passing it to the network layer for routing and transmission across the network. This aggregation allows for efficient and organized transmission of data between devices on a network.