You must not exceed the speed limit when overtaking other vehicles.

Correct principle

When you overtake a truck (or any vehicle), you must:

  • Stay within the posted speed limit at all times.
  • Indicate in advance so other road users know your intention.
  • Check mirrors and blind spots before moving out.
  • Make sure there is enough clear road and no oncoming traffic before you commit.

So, if your question is a multiple‑choice style “Which must you NOT do?”, the wrong/unsafe action is:

Exceed the speed limit to overtake.

Even if it might feel “safer” to get past quickly, traffic law and driving‑test guidance are clear that you must not break the speed limit to overtake.

Quick story to remember it

Imagine you’re on a two‑lane highway behind a long truck. You indicate, check mirrors and blind spot, see a long clear gap ahead, and then pull out. You accelerate firmly but stay at or under the speed limit, pass without lingering beside the truck, and then move back in only when you can see both its headlights in your mirror.

Now imagine instead that you floor it, shooting 20 km/h over the limit to “get it over with.” If something appears ahead or the truck speeds up downhill, you have less time and space to react, and you’re also breaking the law.

Key do’s and don’ts (overtaking trucks)

  • Do check ahead for enough clear distance and oncoming traffic.
  • Do signal early and clearly, then check mirrors and blind spots.
  • Do pass decisively without lingering alongside the truck or cutting in too closely.
  • Do wait and abort the overtake if conditions are not clearly safe.
  • Do not speed above the limit to overtake.
  • Do not pull back in sharply in front of the truck; leave plenty of space.

TL;DR:
For the question “you want to overtake the truck. which of the following must you not do when overtaking other vehicles?”, the correct “must NOT” behaviour is: exceed the speed limit to overtake.

Information gathered from public forums or data available on the internet and portrayed here.