To find the constitutional basis for restricting obscene speech, it helps to go through the grounds listed in Article 19(2) and see which one actually covers obscenity.
Public order covers speech that could cause a breakdown of law and order, riots, or violence. Obscene content does not, by itself, threaten public order.
Security of the State covers speech that threatens the integrity or safety of the nation, such as sedition-adjacent speech or incitement to war. Obscenity has nothing to do with national security.
Defamation covers speech that harms an individual's reputation through false statements. Obscene material is objectionable because of its content and effect on public morals, not because it defames a specific person.
Contempt of court and incitement to an offence are narrow grounds tied to the authority of courts and to provoking criminal acts, neither of which describes obscene material.
Decency or morality is the one ground in Article 19(2) aimed precisely at content that is indecent, vulgar, or morally offensive to the public, which is exactly what obscenity law targets. This is why every obscenity statute in India, from the old Section 292 IPC to Section 67 of the IT Act, is upheld as a reasonable restriction under this specific ground.
Therefore, the correct answer is Article 19(2), restricting speech on the ground of decency or morality.