You are given a passage and ten minutes, and you must summarise it in one sentence of between five and seventy-five words. The form rule is absolute: a full stop in the middle of your answer scores zero on form no matter how good the summary is.
That makes this item almost mechanical, which is good news.
The shape that always works. Take the main idea and the most important supporting point and join them with a conjunction: although, while, because, which, and, but. For example:
"Although urban air quality has improved since the 1990s, traffic remains the largest single source of particulate pollution in most European cities, which is why several governments are now restricting diesel vehicles in city centres."
One sentence. Three clauses. Main idea, supporting point, consequence.
Where the marks are. Content (did you capture the main idea), form (one sentence, 5–75 words), grammar and vocabulary. Not style. A plain sentence that captures the argument beats an elegant one that captures half of it.
Method, in four steps. Read the passage once for the main idea. Underline the topic sentence of each paragraph. Write the main idea as a clause. Attach the one supporting point that matters most using a conjunction. Then count your words and check for a stray full stop.
Two traps. Semicolons are safer than full stops but still risky — use a conjunction instead. And do not copy a whole sentence from the passage: it scores badly on content because it shows no synthesis, even though it is technically one sentence.