Complete the sentence by choosing the phrasal verb that best fits the context. The sentence is: We should first collect the funds and then _____ action.

Introduction / Context:
This question tests knowledge of phrasal verbs and natural English expressions. The sentence describes a sequence of activities: first raising funds and then energetically starting some work or project. A common idiomatic expression for starting energetic activity is to plunge into action. Learners must recognise which preposition correctly completes the phrasal verb plunge into in this context, since prepositions often determine whether a phrase sounds natural and grammatically correct.

Given Data / Assumptions:

• Sentence: We should first collect the funds and then _____ action.
• Options: plunge into, plunge onto, plunge at, plunge in.
• We assume that the sentence describes beginning an activity with energy after preparation.
• The task is to find the phrasal verb that fits both grammatically and idiomatically with action.

Concept / Approach:
The verb plunge literally means to dive or fall suddenly, often into water. In a figurative or idiomatic sense, it means to begin an activity suddenly and with enthusiasm or energy. The usual collocation in English is plunge into an activity, such as plunge into work, plunge into study, or plunge into action. Other prepositions like onto, at, or in do not create the standard idiomatic expression here. The approach is to recall this established pattern or to eliminate options that do not form familiar or correct combinations with action.

Step-by-Step Solution:
Step 1: Read the full sentence with each option to sense which one sounds natural. Step 2: Try plunge into: We should first collect the funds and then plunge into action. This expresses starting activity energetically. Step 3: Try plunge onto: We should first collect the funds and then plunge onto action. This sounds incorrect in standard English. Step 4: Try plunge at: We should first collect the funds and then plunge at action. This also does not form a meaningful phrase. Step 5: Try plunge in: while plunge in can be used in some contexts, plunge in action is not a standard collocation. The usual structure is plunge into action. Step 6: Conclude that plunge into is the only option that correctly completes the phrase.

Verification / Alternative check:
To verify, consider other common uses of this phrasal verb. Phrases such as the team plunged into preparations for the event, after the delay we must plunge into work, and once the funds arrive they will plunge into the project are widely used and accepted. In each of these, into appears immediately after plunge. The expression plunge into action is also frequently used in news and literature to describe people who start doing something energetic at once. This consistent pattern confirms that plunge into is the correct choice for the sentence in the question.

Why Other Options Are Wrong:
plunge onto: This preposition suggests movement onto a surface and is not used with abstract nouns like action in this meaning.
plunge at: This could suggest attacking or rushing at something physically, but it does not combine naturally with action in this context of starting work.
plunge in: Although plunge in can be used informally meaning join suddenly, it normally requires an object or context, and the standard idiom with action specifically is plunge into action, not plunge in action.

Common Pitfalls:
Phrasal verbs are challenging because small changes in preposition can completely alter the meaning or make the phrase incorrect. Learners sometimes rely on guesswork based on literal meanings of prepositions, which does not always work in idiomatic expressions. To avoid this, it is helpful to learn phrasal verbs as fixed combinations, such as plunge into action, jump at an opportunity, and throw oneself into work. Regular reading and exposure to authentic English helps internalise these expressions.

Final Answer:
The correct phrase is plunge into, so the sentence should read: We should first collect the funds and then plunge into action.