Option 1 states that a part of India`s national wealth or total annual product was being exported to Britain without receiving any material returns. This suggests that India`s resources were being taken away without any benefit to India itself.

Option 2 defines the drain theory more accurately. It states that the resources of India were being utilized in the interest of Britain. This means that Britain was exploiting India`s resources for its own benefit, leading to a drain of wealth from India.

Option 3 suggests that the British industrialists were given an opportunity to invest in India under the protection of the imperial power. While this may have been true to some extent, it does not accurately capture the essence of the drain theory as proposed by Dadabhai Naoroji.

Option 4 states that British goods were being imported to India, making the country poorer day by day. While this may have been a consequence of the drain theory, it does not directly explain the theory itself.

Overall, option 2 provides the most accurate definition of the drain theory as propounded by Dadabhai Naoroji, highlighting the exploitation of India`s resources for Britain`s benefit.