The answer is option B: Carnot cycle and Stirling cycle.

Explanation:

Let's analyze each pair of cycles to determine which ones have at least one isothermal process:

a) Bell-Coleman cycle and Vapour compression refrigeration cycle:
- The Bell-Coleman cycle is a refrigeration cycle that consists of four processes: compression, cooling, expansion, and heating. None of these processes are isothermal, as they involve changes in temperature.
- The Vapour compression refrigeration cycle is also a refrigeration cycle that consists of four processes: compression, condensation, expansion, and evaporation. None of these processes are isothermal either.

b) Carnot cycle and Stirling cycle:
- The Carnot cycle is an idealized thermodynamic cycle that consists of four reversible processes: isothermal expansion, adiabatic expansion, isothermal compression, and adiabatic compression. The two isothermal processes in this cycle occur at the highest and lowest temperatures of the cycle.
- The Stirling cycle is a thermodynamic cycle that operates on a closed loop and consists of four processes: isothermal expansion, constant volume cooling, isothermal compression, and constant volume heating. The two isothermal processes in this cycle occur at the same temperatures as the Carnot cycle.

c) Brayton cycle and Rankine cycle:
- The Brayton cycle is a thermodynamic cycle that describes the operation of gas turbine engines and consists of four processes: isentropic compression, constant-pressure heat addition, isentropic expansion, and constant-pressure heat rejection. None of these processes are isothermal.
- The Rankine cycle is a thermodynamic cycle that describes the operation of steam power plants and consists of four processes: isentropic compression, constant-pressure heat addition, isentropic expansion, and constant-pressure heat rejection. None of these processes are isothermal either.

d) Diesel cycle and Otto cycle:
- The Diesel cycle is a thermodynamic cycle that describes the operation of diesel engines and consists of four processes: isentropic compression, constant-pressure heat addition, isentropic expansion, and constant-volume heat rejection. None of these processes are isothermal.
- The Otto cycle is a thermodynamic cycle that describes the operation of spark-ignition engines and consists of four processes: isentropic compression, constant-volume heat addition, isentropic expansion, and constant-volume heat rejection. None of these processes are isothermal either.

Therefore, the only pair of cycles that have at least one isothermal process is the Carnot cycle and Stirling cycle (option B).