Final answer:
The blank in the sentence should be filled with the word 'maintained,' signifying that a person's skill level has remained steady without deterioration.
Step-by-step explanation:
The sentence to be completed here is: Someone who has maintained their ability to do something has not gotten worse at it. The word maintained indicates that an individual has kept their level of skill or ability constant over time without decline. This statement implies consistency and ongoing proficiency in a particular skill or area. . For example, if a student works hard and practices regularly, they may improve their reading comprehension skills or their ability to solve math problems.