The correct answer is option 3 - "As long as he enjoys the support of a majority in the Lok Sabha."

Let`s dissect each option to understand why option 3 is correct and the others are incorrect:

Option 1 states that the tenure of the Prime Minister is conterminous with the tenure of the Lok Sabha. This means that the Prime Minister`s term will end when the Lok Sabha`s term ends. However, this is not accurate, as the Prime Minister can continue in office even if the Lok Sabha is dissolved before completing its full term.

Option 2 suggests that the tenure of the Prime Minister is conterminous with the tenure of the President. This is incorrect because the President and the Prime Minister have independent tenures. The President`s tenure is five years, while the Prime Minister`s tenure depends on the support of the majority in the Lok Sabha.

Option 4 states that the tenure of the Prime Minister is five years. However, this is also incorrect because the Prime Minister`s tenure is not fixed and can vary based on the political circumstances and support in the Lok Sabha.

Therefore, option 3 is the correct answer as it accurately represents the tenure of the Prime Minister, which is dependent on the majority support in the Lok Sabha.