The principle of "equality before law" in India is derived from the Constitution. The concept of equality before law is a fundamental principle that ensures that every citizen is treated equally under the law without any discrimination.

Option 1, case law, refers to the principles and judgments established by courts in previous cases. While case law can contribute to the interpretation and application of the principle of equality, it is not the source of this principle.

Option 2, political conventions, are informal rules and customs that govern political behavior. While political conventions may play a role in shaping laws, they are not the source of the principle of equality before law.

Option 4, Gandhian philosophy, promotes principles such as non-violence and self-reliance but is not directly related to the principle of equality before law.

Therefore, the correct answer is option 3, the Constitution. The Indian Constitution explicitly guarantees equality before law in Article 14.