Ignition system practice — the typical spark plug gap between the central electrode and the ground electrode in a conventional petrol engine is approximately:

Introduction / Context:
The spark plug gap is the distance the ignition system must arc across to ignite the compressed air-fuel mixture. The correct gap balances reliable ignition at various pressures with coil energy limits and erosion considerations. Too small a gap can lead to weak ignition; too large a gap can misfire under high cylinder pressure, especially during acceleration or cold starts.

Given Data / Assumptions:

• Conventional gasoline engine with standard ignition energy levels.
• No specialty racing or extended-spark systems are assumed.
• General workshop settings rather than vehicle-specific extremes.

Concept / Approach:

Typical automotive spark plug gaps fall around 0.5 to 0.9 mm depending on ignition system capability, combustion chamber design, and emissions strategy. A commonly taught nominal value for standard plugs and mid-range systems is about 0.5 mm, which provides reliable ignition margin without excessive coil demand. While some modern systems specify larger gaps closer to 0.7–0.9 mm, the option set provided places 0.5 mm as the most representative textbook value.

Step-by-Step Solution:

Verification / Alternative check:

Service manuals often list gaps from 0.5 to 0.8 mm; older breaker-point systems commonly used around 0.5 mm, reinforcing the choice here.

Why Other Options Are Wrong:

0.2 mm risks weak spark kernel and fouling tolerance; 1 mm or 1.5 mm demand high voltage that many systems cannot supply under load, risking misfire.

Common Pitfalls:

Setting the gap without considering coil capability and boost or high compression effects; forgetting that erosion widens the gap over service life.

Final Answer:

0.5 mm