The correct answer is 1.) A substance that gives off light
Explanation:
The word "phosphorus" is composed of the root word "phos" that derives from Greek and means light and the suffix "phoros" that means to emit or bring. This implies according to its root and composition "phosphorus" refers to something that emits light. Besides this, this word is used to describe an element or substance involved in a chemical reaction "we observed the reaction when phosphorus is exposed to oxygen"; this suggests phosphorus is a chemical compound or substance. Thus, according to its root and use in the sentence "phosphorus" is "a substance that gives off light".
The italicized word or phrase <em>a three-pronged </em>(if this is italicized, we cannot possibly know) is attributive, as it is an adjective to describe the plan.
Present perfect tense. Have washed is the verb, Have or Has is used to show an action that has taken place in the present, not quite long or is still taking place
