Final answer:
1) Wood. The word that best completes the sentence is wood.
Step-by-step explanation:
The word that best completes the sentence is wood. Wood is a common material used for railings, especially in residential settings. It is strong, durable, and provides a traditional look. Metal, glass, and plastic are also possible materials for railings, but they may not be as suitable as wood in terms of strength and aesthetic appeal for a staircase railing.