Safer a city is, the more liveable it becomes.

Mistake: A. Safer
Correction: The safer a city is, the more liveable it becomes.
Explanation: The article "The" is needed before "safer" in comparative constructions.

Our city is age-friendly. We make it easy for the elderly to get along by public transport.

Mistake: D. get along
Correction: Our city is age-friendly. We make it easy for the elderly to get around by public transport.
Explanation: "Get around" is the correct phrase when talking about movement or travel, not "get along."

The bigger the shopping mall is, the attractive it is to teenagers.

Mistake: C. attractive
Correction: The bigger the shopping mall is, the more attractive it is to teenagers.
Explanation: In comparative constructions, "more" is needed before adjectives like "attractive."

Although people wanted to carry out with the project, the company stopped funding it.

Mistake: B. out
Correction: Although people wanted to carry on with the project, the company stopped funding it.
Explanation: "Carry on" is the correct phrasal verb meaning to continue something.

The local authority tried to cut on private car use but they didn’t succeed.

Mistake: B. cut on
Correction: The local authority tried to cut down on private car use but they didn’t succeed.
Explanation: The correct phrase is "cut down on," meaning to reduce something.

The closer an area is to the downtown, denser its population is.

Mistake: C. denser
Correction: The closer an area is to the downtown, the denser its population is.
Explanation: The comparative structure requires "the" before the second adjective.

Many teens prefer hanging in at shopping malls to outdoor places.

Mistake: C. at
Correction: Many teens prefer hanging out at shopping malls to outdoor places.
Explanation: The correct phrase is "hanging out" when referring to spending time leisurely.

Hoàng is coming up with a fever. The more time that passes, the higher his temperature is.

Mistake: A. up with
Correction: Hoàng is coming down with a fever. The more time that passes, the higher his temperature is.
Explanation: "Coming down with" is the correct phrase for beginning to get sick.