Meaning: Identical means being exactly alike or the same in every way.

Wrong options:

• eligible: 'Eligible' is an adjective that refers to being qualified or entitled, which does not apply to the context of describing the glasses.
• separate: 'Separate' means distinct or not together, which contradicts the idea of the glasses being the same for the experiment.
• independent: 'Independent' refers to being free from outside control, which does not fit as it does not describe the similarity of the glasses.
• emergence: 'Emergence' refers to the process of coming into view or becoming exposed, which is irrelevant to the description of the glasses.

Correct answer: introduction

Explanation: The term 'introduction' refers to the act of bringing in the ink as a new factor in the experiment. It fits grammatically as a noun and contextually as it describes the action that leads to the observed change in color of the water.

Meaning: Introduction means the act of bringing something into a situation or environment.

Wrong options:

• identical: 'Identical' does not fit here as it describes similarity, not the action of adding a new factor.
• eligible: 'Eligible' refers to being qualified, which does not relate to the context of the experiment.
• separate: 'Separate' implies distinction, which does not apply to the action of adding a factor in this context.
• independent: 'Independent' refers to a variable that is not affected by others, which does not describe the action of introducing something new.
• emergence: 'Emergence' refers to coming into view, which does not accurately describe the act of adding the ink.

Correct answer: independent

Explanation: The word 'independent' is used to describe the variable that is manipulated in an experiment to observe its effect. It fits grammatically as an adjective modifying 'variable' and contextually as it identifies the ink as the factor being tested in the experiment.

Meaning: Independent refers to a variable that is changed or controlled in a scientific experiment to test its effects on the dependent variable.

Wrong options:

• identical: 'Identical' describes similarity, which does not apply to the variable being manipulated in this context.
• eligible: 'Eligible' refers to being qualified, which does not relate to the concept of variables in an experiment.
• separate: 'Separate' implies distinction, which does not fit as it does not describe the nature of the variable being tested.
• introduction: 'Introduction' refers to the act of bringing in a new factor, not the type of variable itself.
• emergence: 'Emergence' refers to the process of coming into view, which does not accurately describe the nature of the variable.