Trailer is only added at layer of osi model.
a. data link layer
b. physical layer
c. network layer
d. transport layer

a. Data link layer

Explanation:

A trailer / footer is the additional data (metadata) which is positioned at the end of the data block being stored or transferred,it  may contain information for the processing of the data block or simply placed at the end of the block.

In data transmission, the data is called the payload or body after the end of the header and before the trailer starts.

In Data link layer, it contain three parts-

answerequal fluid intake and fluid loss;