To solve this question, the user needs to have an understanding of the basic components and terminology used in Fibre Channel (FC) networks.

In Fibre Channel, data is transferred in units called "frames." A frame is the smallest unit of information transfer in FC. It contains the data being transmitted as well as control information such as headers and CRC (Cyclic Redundancy Check) for error detection.

With this knowledge, let's evaluate each option:

A. Block: A block is a unit of data storage and is not the smallest unit of information transfer in FC. It refers to a set of data, typically a fixed size, that is read from or written to storage devices.

B. Frame: This option is correct. In Fibre Channel, a frame is the smallest unit of information transfer. It contains the data being transmitted along with control information.

C. Cylinder: A cylinder is a term used in disk storage and refers to a group of tracks that are located at the same radial distance from the center of the disk. It is not the smallest unit of information transfer in FC.

D. File: A file is a collection of data stored on a file system and is not the smallest unit of information transfer in FC.

Therefore, the correct answer is:

B. Frame