India is a republic because the head of the state is elected for a definite period. In a republic, the head of the state is not a hereditary monarch but an elected official. This is in contrast to a monarchy where the head of the state is a king or a queen who inherits the position.

Option 1, stating that India is a democratic country, is not the sole reason why India is a republic. Many democratic countries have a monarchy as the head of the state, such as the United Kingdom.

Option 2, mentioning that India is a parliamentary democracy, is not directly related to the concept of a republic. A parliamentary democracy refers to a system where the executive branch of the government is accountable to the legislature.

Option 4, stating that all of the options are correct, is incorrect. Only option 3 is the reason why India is a republic.