Here's why:
* Letter refers to the written communication itself, which is a concept or idea, not a physical object. It's something intangible.
Let's break down the other words:
* Tattered is an adjective describing the envelope.
* Envelope is a concrete noun, referring to the physical container.
* Held is a verb.
* The is a definite article.
* From is a preposition.
* My is a possessive adjective.
* Sister is a concrete noun, referring to a person.
Only "letter" represents an abstract concept.