The main reason synthetic rubber has replaced natural rubber for domestic and industrial purposes is that natural rubber is unable to meet the growing demand of different industries. This means that the demand for rubber is increasing, but natural rubber production cannot keep up with the demand. Synthetic rubber, on the other hand, can be produced in larger quantities to meet the needs of various industries.

Option 2, that natural rubber is grown in tropical countries only, is not the main reason for the replacement of natural rubber. While it is true that natural rubber production is primarily concentrated in tropical regions, this does not directly explain why synthetic rubber has replaced it.

Option 3, that the raw material for synthetic rubber is easily available, does support the argument for the replacement of natural rubber. If the raw materials for synthetic rubber are readily accessible, it becomes easier and more cost-effective to produce synthetic rubber compared to natural rubber.

Option 4, that natural rubber is not durable, is not a correct reason for the replacement of natural rubber. Both natural and synthetic rubber can be formulated to have varying levels of durability, so this is not the main factor driving the shift towards synthetic rubber.

In conclusion, the main reason for the replacement of natural rubber with synthetic rubber is that natural rubber is unable to meet the growing demand