The correct answer is "1 and 2 only."

In the parliamentary government system of India, the following principles are implied institutionally:

1. Members of the Cabinet are Members of the Parliament: This means that the ministers who are appointed to the Cabinet are also elected representatives and members of either the Lok Sabha or the Rajya Sabha.

2. Ministers hold office till they enjoy confidence in the Parliament: The ministers in the Cabinet hold their positions as long as they have the support and confidence of the majority of the members in the Parliament. If they lose the confidence of the Parliament, they may be removed from their positions.

3. Cabinet is headed by the Head of the State: This statement is incorrect. In the parliamentary government system of India, the Cabinet is headed by the Prime Minister, who is the Head of the Government. The President of India, who is the Head of the State, is a separate constitutional position and does not directly head the Cabinet.

Therefore, the correct answer is "1 and 2 only."