Commercial vulcanisation of rubber involves the use of sulphur. Vulcanisation is a process in which raw rubber is treated with sulphur to improve its properties.

Option 1: Sulphur - This is the correct answer. Sulphur is the main ingredient used in the vulcanisation process of rubber. It forms cross-links between the polymer chains of rubber, making it stronger, more durable, and resistant to heat, chemicals, and weathering.

Option 2: Carbon - Carbon is not directly involved in the vulcanisation process of rubber. However, carbon black is commonly added to rubber compounds to improve their strength, abrasion resistance, and overall performance.

Option 3: Carbon phosphorus - This option is incorrect. Phosphorus is not typically used in the vulcanisation process of rubber.

Option 4: Selenium - Selenium is not used in the commercial vulcanisation process of rubber. It is mainly used in electronic components, glass manufacturing, and photocopiers.

In conclusion, the correct answer is option 1 - sulphur. It is the key ingredient used in the commercial vulcanisation process of rubber.