Select the option that can be used as a one-word substitute for the given group of words/phrase.

Cause to continue; keep up; preserve (a state of affairs, an activity etc)

maintain

The word that can substitute the given group of words/phrase is maintain.

• Maintop is the topmost mast on a sailing ship.
• Mainly means for the most part or on the whole.
• Maintain means to cause to continue; keep up; preserve (a state of affairs, an activity etc).
• Mainstay means a principal support or prop.

Maintain is the most appropriate word for the given group of words/phrase because it specifically refers to the action of keeping something going or in existence. It is derived from the Old French word maintenir, meaning "to hold in place."