In the following sentence completion question, choose the most appropriate verb to fill in the blank: The vehicles involved in the accident were ________ the flow of traffic.

Introduction / Context:
This sentence completion question focuses on selecting the most appropriate verb to describe the effect of vehicles involved in an accident on the flow of traffic. The correct answer needs to fit both semantically and collocationally, that is, it should be a word that naturally combines with the phrase the flow of traffic in standard English usage. Understanding typical verb noun combinations in traffic related contexts is important here.

Given Data / Assumptions:

• Sentence: The vehicles involved in the accident were ________ the flow of traffic.
• Options: obstructing, stalling, interfering, delaying.
• We assume a typical road situation where an accident leads to disruption of normal vehicle movement.
• The focus is on continuous action, indicated by were plus present participle.

Concept / Approach:
The phrase the flow of traffic is commonly used with verbs such as obstruct, disrupt, or block. Among the given options, obstructing directly expresses the idea of blocking or hindering movement. While other verbs may relate loosely to causing problems, they do not combine as strongly with the phrase the flow of traffic or they introduce a slightly different nuance. Hence, we focus on collocation and precise meaning in this context.

Step-by-Step Solution:
Step 1: Consider the overall meaning: vehicles after an accident are physically present on the road and prevent normal movement. Step 2: Examine obstructing, which means blocking the way or preventing something from moving freely. Step 3: Insert obstructing into the sentence: The vehicles involved in the accident were obstructing the flow of traffic. This sounds natural and precise. Step 4: Examine stalling, which usually refers to engines stopping or progress being delayed, and is more often used with vehicles themselves rather than the flow of traffic as an abstract noun. Step 5: Examine interfering, which means getting in the way of, but is less commonly used with the noun phrase the flow of traffic, and tends to be used more widely in abstract contexts. Step 6: Examine delaying, which focuses on causing lateness rather than directly blocking movement, and is more commonly used with people or events rather than the flow of traffic itself. Step 7: Conclude that obstructing is the best fit for both meaning and collocation.

Verification / Alternative check:
A quick check against typical usage confirms this choice. News reports and traffic bulletins frequently say that an accident is obstructing traffic or obstructing the flow of traffic. Other possible verbs like blocking would also fit, but blocking is not one of the options. None of the other choices produces such a natural phrase when combined with the flow of traffic. Therefore, obstructing is clearly the most appropriate word among the alternatives given.

Why Other Options Are Wrong:
Stalling is wrong because it suggests that the vehicles themselves are stopping or failing to function, but the sentence emphasises the effect on the flow of traffic. Interfering is not completely wrong in meaning but is not the most idiomatic choice with this specific noun phrase. Delaying is incorrect because it primarily relates to postponing or slowing in terms of time rather than literally blocking movement, and it is not the standard collocation here. These alternatives fail to capture the direct physical blockage implied by the situation.

Common Pitfalls:
Many test takers look only at approximate meanings and ignore how words are actually used together. They might pick interfering or delaying because they think of general disturbance or lateness, but they miss the strong collocation between obstructing and flow of traffic. To improve performance on such questions, learners should pay attention to common phrases in newspaper reports, official notices, and announcements related to transport and public services.

Final Answer:
The vehicles involved in the accident were obstructing the flow of traffic.