Introduction:
Polyol ester is a type of lubricant commonly used in refrigeration systems. It is known for its excellent lubrication properties, chemical stability, and compatibility with refrigerants. In this question, we are asked to identify the suitable refrigerant for a system using modified polyol ester as the lubricant. The correct answer is option 'B', which is R134a.

Explanation:
To understand why R134a is the suitable refrigerant for a system using modified polyol ester as the lubricant, let's look at the properties of polyol ester lubricants and the refrigerants mentioned in the options.

Polyol Ester Lubricants:
- Polyol ester lubricants are synthetic lubricants that have excellent lubrication properties, including high film strength and low friction.
- They have good chemical stability, which helps in preventing the breakdown of the lubricant under high temperatures and pressures.
- Polyol ester lubricants also have good compatibility with a wide range of refrigerants, making them suitable for various refrigeration systems.

R11:
- R11 is a chlorofluorocarbon (CFC) refrigerant that has been phased out due to its ozone depletion potential.
- It is not commonly used in modern refrigeration systems.

R134a:
- R134a is a hydrofluorocarbon (HFC) refrigerant that is commonly used as a replacement for CFCs.
- It is non-toxic, non-flammable, and has zero ozone depletion potential.
- R134a has a low boiling point and is suitable for a wide range of temperatures, making it suitable for various applications, including automotive air conditioning and refrigeration systems.
- It is compatible with polyol ester lubricants and exhibits good lubricity.

R245fa:
- R245fa is a hydrofluorocarbon (HFC) refrigerant that is commonly used in low-temperature applications, such as chillers and heat pumps.
- It has a relatively high boiling point and is suitable for applications requiring lower temperatures.
- While R245fa is compatible with polyol ester lubricants, it is not the most suitable refrigerant for a system using this type of lubricant.

R12:
- R12 is a chlorofluorocarbon (CFC) refrigerant that has been phased out due to its ozone depletion potential.
- It is not commonly used in modern refrigeration systems.

Conclusion:
Based on the properties of polyol ester lubricants and the refrigerants mentioned in the options, the most suitable refrigerant for a system using modified polyol ester as the lubricant is R134a. R134a is compatible with polyol ester lubricants and exhibits good lubricity. It is also a widely used refrigerant in various applications, making it a suitable choice for refrigeration systems.