Meaning: A 'phenomenon' is an observable fact or event, particularly one that is unusual or significant.

Wrong options:

• event: While 'event' can refer to an occurrence, it lacks the specificity and scientific connotation that 'phenomenon' carries in this context.
• removal: 'Removal' is a noun that refers to the act of taking something away, which does not describe the nature of riverbank erosion.
• basal: 'Basal' is an adjective that describes something related to the base, which does not fit as a noun for the context of erosion.
• migration: 'Migration' refers to the movement from one place to another, which is unrelated to the concept of erosion as a natural occurrence.
• additional: 'Additional' is an adjective that means extra or supplementary, which does not fit the context of describing riverbank erosion.

Correct answer: removal

Explanation: The term 'removal' is a noun that describes the action of taking away materials from the river banks, which is central to the process of bank erosion. It fits grammatically as the object of the verb 'involves.'

Meaning: 'Removal' refers to the act of taking something away or eliminating it, in this case, bank materials.

Wrong options:

• phenomenon: 'Phenomenon' is a noun that describes an observable event, but it does not fit as an action related to bank erosion.
• basal: 'Basal' is an adjective that describes something related to the base, which does not fit as a noun for the action of erosion.
• migration: 'Migration' refers to movement, which does not describe the action of taking away materials from the banks.
• additional: 'Additional' is an adjective that means extra, which does not fit as a noun describing the action involved in erosion.

Correct answer: basal

Explanation: The word 'basal' is an adjective that describes the foundational support of the riverbank. It fits grammatically as it modifies the noun 'support' and is contextually relevant to the discussion of erosion.

Meaning: 'Basal' refers to something that is at the base or foundation of something else.

Wrong options:

• phenomenon: 'Phenomenon' is a noun that does not fit as a descriptor for the type of support being discussed in the context of erosion.
• removal: 'Removal' is a noun that refers to the act of taking away, which does not describe the type of support in this context.
• migration: 'Migration' refers to movement, which does not relate to the concept of support in the context of erosion.
• additional: 'Additional' is an adjective that means extra, which does not fit as a descriptor for the type of support in this context.