What is in intaglio ink on the E passport?

The correct answer is that intaglio ink on the E passport serves as a security feature. Intaglio printing involves engraving patterns or texts that provide a tactile experience, enhancing the document's security by making it difficult to replicate. This printing method is commonly used in passports to include various elements that deter counterfeiting and forge attempts.

The use of intaglio ink specifically attributes to features that can include intricate designs, seals, or other identifiers that are hard to duplicate accurately without the right technology. While elements like the national anthem, the eagle seal, or the holder's signature might appear inside the passport, they are not fundamentally associated with intaglio printing as a security feature. Instead, the focus is on how the intaglio ink contributes to the integrity and protection of the passport itself.