Answer: Efficient

## Explanation This question tests the concept of **efficiency** in statistical estimation. **Key definitions:** - **Unbiasedness**: An estimator is unbiased if its expected value equals the true parameter value. - **Efficiency**: Among all unbiased estimators, the one with the smallest variance is called efficient. - **Consistency**: An estimator is consistent if it converges in probability to the true parameter value as sample size increases. - **Reliability**: This is not a standard statistical property of estimators. Since the question specifically states that the estimator has "smaller variance than all other unbiased estimators," this is the definition of an **efficient estimator**. An estimator can be unbiased but not efficient if other unbiased estimators exist with smaller variance.

Author: LeetQuiz Editorial Team