Austempering is a heat treatment process that is used to produce bainitic microstructure in ferrous alloys. The process involves heating the material to a temperature within the austenitic region and then rapidly quenching it in a medium such as oil or a salt bath. The material is then held at a temperature in the range of 250-400°C for a period of time, depending on the material and the desired microstructure.

The austempering process results in a bainitic microstructure that is characterized by a fine, needle-like structure. This microstructure provides excellent strength and toughness properties, making it ideal for applications in which high strength and toughness are required.

The other options given in the question are not possible outcomes of the austempering process. The reasons for this are as follows:

- Option A: 100% martensitic structure cannot be obtained through austempering because the rapid quenching step is not sufficient to fully transform the austenite into martensite. Instead, the material will transform into a mixture of bainite and residual austenite.
- Option C: A 50/50 mixture of martensite and bainite cannot be obtained through austempering because the process is designed to produce a bainitic microstructure. To obtain a mixture of martensite and bainite, a different heat treatment process would be required.
- Option D: 100% pearlitic structure cannot be obtained through austempering because the process involves rapid quenching, which is not conducive to the formation of pearlite. Instead, the material will transform into a bainitic microstructure.

Therefore, the correct answer is option B: 100% bainitic structure can be obtained through austempering.