Meaning: 'Discharging' refers to the act of releasing or emitting energy, in this case, the energy of lightning.

Wrong options:

• discard: 'Discard' is a verb meaning to throw away, which does not relate to the action of lightning and does not fit grammatically in this context.
• amount: 'Amount' is a noun that refers to a quantity, which does not fit as it does not describe an action related to lightning.
• containing: 'Containing' is a gerund that implies holding something within, which does not relate to the action of lightning discharging and does not fit the sentence structure.

Correct answer: mechanisms

Explanation: The term 'mechanisms' is a plural noun that fits the context of discussing the various processes involved in lightning. It aligns with the sentence structure and indicates that multiple processes remain a mystery.

Meaning: 'Mechanisms' refers to the processes or systems that explain how something works, in this case, the processes of lightning.

Wrong options:

• discard: 'Discard' is a verb that means to throw away, which does not fit as it does not refer to processes or systems.
• amount: 'Amount' is a noun that refers to a quantity, which does not fit as it does not describe processes related to lightning.
• containing: 'Containing' is a gerund that implies holding something within, which does not relate to the processes of lightning.

Correct answer: untangle

Explanation: The verb 'untangle' fits grammatically as it describes the action researchers are taking to clarify the complexities of lightning. It aligns with the context of trying to understand the mystery behind lightning's behavior.

Meaning: 'Untangle' means to free from confusion or complexity, in this case, referring to clarifying the reasons behind lightning's zigzag pattern.

Wrong options:

• discard: 'Discard' is a verb meaning to throw away, which does not fit as it does not relate to the action of understanding or clarifying.
• amount: 'Amount' is a noun that refers to a quantity, which does not fit as it does not describe an action related to research.
• containing: 'Containing' is a gerund that implies holding something within, which does not relate to the action of clarifying or understanding.

Correct answer: characteristic

Explanation: The word 'characteristic' is an adjective that describes the zigzag pattern of lightning. It fits grammatically and contextually as it indicates a defining feature of lightning's appearance.

Meaning: 'Characteristic' refers to a distinguishing feature or quality, in this case, the zigzag pattern of lightning.

Wrong options:

• discard: 'Discard' is a verb meaning to throw away, which does not fit as it does not describe a feature of lightning.
• amount: 'Amount' is a noun that refers to a quantity, which does not fit as it does not describe a quality or feature.
• containing: 'Containing' is a gerund that implies holding something within, which does not relate to describing the pattern of lightning.