In each of the following sentences, a word is missing and replaced by a blank.…

Concept

This is a contextual word-choice task: pick the word whose meaning matches what the sentence describes, so the intended sense is preserved. The decisive clues are the cause and the result inside the sentence itself — here, a switch being flipped (the cause) and the machine then operating smoothly (the result). The right word must name the change the switch produces: turning the device on.

Application

Read the sentence with each candidate and keep only the word whose meaning fits both the cause and the result:

• activated — means “made operational / switched on.” Flipping a switch turns a machine on, and “began operating smoothly” confirms it is now running. The meaning is preserved exactly.

• deactivated — means “switched off.” This reverses the sentence: a machine cannot be turned off and “begin operating” in the same breath, so it breaks the cause–result link.

• invoked — means “called upon / cited” (a rule, a name, a procedure). It is not used for powering a physical machine.

• captivated — means “fascinated / held the attention of.” It applies to people’s interest, not to a machine starting up.

• weltered — means “rolled about / lay soaked.” It has no link to switching on equipment.

Cross-check

Substitute the chosen word back: “the machine was activated and began operating smoothly” reads naturally and keeps the on→running logic intact, while every other word either contradicts the result or misnames the action.

Result

The word that fits without altering the meaning is activated.