**Core Concept**
Transport across the nuclear envelope is a critical process that regulates the exchange of molecules between the nucleus and the cytoplasm. The nuclear envelope is a double membrane structure that consists of the outer nuclear membrane and the inner nuclear membrane, which are separated by the perinuclear space. There are specific mechanisms that allow for the transport of molecules across this barrier.

**Why the Correct Answer is Right**
The transport across the nuclear envelope occurs primarily through nuclear pores, which are protein channels that span the nuclear envelope. The nuclear pores are composed of several subunits, including the nucleoporins, which form the central channel and the annulus. The transport of molecules across the nuclear envelope involves the selective movement of proteins, RNA, and other molecules through the nuclear pores. This process is facilitated by the nuclear pore complex, which recognizes and interacts with specific signals on the molecules being transported.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe a mechanism of transport across the nuclear envelope.
**Option B:** This option is incorrect because it does not accurately describe a mechanism of transport across the nuclear envelope.
**Option C:** This option is incorrect because it does not accurately describe a mechanism of transport across the nuclear envelope.

**Clinical Pearl / High-Yield Fact**
The nuclear pore complex is a critical regulator of gene expression, and defects in nuclear pore function have been implicated in several diseases, including cancer and neurodegenerative disorders.

**Correct Answer:** D.