In each of the following questions, a word has been

Solution

"Sustain" means to maintain or continue something over a period of time, especially when it requires effort, support, or resilience. It can also mean to endure or support in challenging situations. (I) "The company’s ability to sustain its market share despite economic downturns is a testament to its strong leadership and innovative strategies."--Correct usage Here, "sustain" is used to describe the company’s ability to maintain or keep its market share over time, even during challenging conditions. (II) "The athlete struggled to sustain his performance throughout the race, but his perseverance helped him finish strong."--Correct usage In this sentence, "sustain" refers to the ability to maintain a level of performance over a long period. (III) "The doctor was pleased to see that the patient’s condition was improving and that she could sustain the prescribed treatment plan without complications." -- Correct usage Here, "sustain" is used to show that the patient can continue following the prescribed treatment without any issues, meaning that the patient can endure or maintain the treatment regimen.