The correct answer is option 4, "Rustlessness." The Iron Pillar near Qutub Minar is a unique archaeological structure in Delhi, India, that has drawn the attention of scientists. It is known for its remarkable rust-resistant quality, as it has remained relatively free from corrosion for centuries. This is a significant feat considering that iron tends to rust when exposed to moisture and air.

Option 1, "Antiquity," is not the main reason why the Iron Pillar draws attention. While its age is certainly impressive (dating back to the 4th century AD), scientists are particularly interested in its rust resistance.

Option 2, "Glitter," is not a factor that attracts scientists to the Iron Pillar. It is made of iron, which does not have a shiny or glittery appearance.

Option 3, "Hardness," although a noteworthy characteristic of the Iron Pillar, is not the primary reason why it draws scientific attention. Hardness is a general property of iron, but its resistance to rust sets it apart and intrigues scientists.

Therefore, the correct answer is option 4, "Rustlessness," as it highlights the unique feature of the Iron Pillar that captivates scientists.