Correct Answer: Capability

Solution : The correct choice is the fourth option.

Efficiency refers to the ability to achieve a goal or perform tasks effectively with minimal waste of time and effort. Among the given options:

• Disability: This implies a lack of ability, which is the opposite of efficiency.
• Fluency: Typically relates to language skills, not general effectiveness or efficiency.
• Tendency: This means a propensity or inclination to behave in a certain way, not related to efficiency.
• Capability: This implies having the skills or qualities to achieve something effectively, which aligns closely with the concept of efficiency.

Therefore, "capability" is the best synonym for "efficiency" in this context.