The correct answer is A. 60 mm.
The permissible limit of flat tyre in goods and coaching vehicles is 60 mm and 50 mm respectively. This means that the tread depth of the tyre must be at least 60 mm for goods vehicles and 50 mm for coaching vehicles. If the tread depth is less than this, the tyre is considered to be flat and must be replaced.
Tread depth is the distance from the top of the tread to the bottom of the grooves in the tyre. The tread helps to grip the road and provide traction, so it is important to keep it in good condition. If the tread depth is too low, the tyre will not be able to grip the road as well and this can lead to accidents.
It is important to check the tread depth of your tyres regularly and to replace them when necessary. You can check the tread depth with a tread depth gauge or a coin. To use a tread depth gauge, simply insert it into the groove of the tyre and measure the depth of the tread. To use a coin, insert the edge of the coin into the groove of the tyre. If the coin is flush with the bottom of the groove, the tread depth is too low and the tyre needs to be replaced.
It is also important to rotate your tyres regularly. This helps to ensure that all of the tyres wear evenly. To rotate your tyres, simply swap the front tyres with the back tyres and the left tyres with the right tyres. You should rotate your tyres every 6,000 to 8,000 miles.
By following these simple tips, you can help to keep your tyres in good condition and avoid accidents.