Final answer:
A. Unsterile instruments. Unsterile instruments are a major factor in contributing to a patient contracting an infection in the hospital.
Step-by-step explanation:
The major factor in contributing to a patient contracting an infection while in the hospital is unsterile instruments.
Unsterile instruments can introduce bacteria and other pathogens into the patient's body, increasing the risk of infection.
For example, if a surgical instrument is not properly sterilized before use, it can contaminate the surgical site and lead to post-operative infections.