Option 1: To practice secularism. This option is not an item included in the list of Fundamental Duties of an Indian citizen in the Constitution. Secularism is a principle followed by the state, but it is not specified as a duty for citizens.

Option 2: To develop scientific temper, humanism and the spirit of enquiry and reform. This option is the correct answer. It is one of the Fundamental Duties of an Indian citizen as mentioned in Article 51A(h) of the Constitution. It emphasizes the importance of promoting a scientific mindset, humanistic values, and the desire to question and bring about positive change.

Option 3: To pay all taxes to the government regularly and correctly. This is not a specific Fundamental Duty mentioned in the Constitution. While paying taxes is an obligation, it is not explicitly listed as a Fundamental Duty.

Option 4: Not to assault any public servant in the performance of his duties. This is not a Fundamental Duty either. It is more of a responsibility to respect public servants, but not listed in the Constitution as a Fundamental Duty.

In conclusion, the correct option is Option 2, to develop scientific temper, humanism, and the spirit of enquiry and reform.