​The movement of proteins bigger than 50 kDa across the nuclear envelope requires:​

Correct option is C

Explanation-

The nuclear envelope surrounds the nucleus and has nuclear pore complexes (NPCs). Small molecules (< 40–50 kDa) can diffuse freely through these pores. Large proteins (>50 kDa) cannot pass freely — they need help.
The large protein can enter the nucleus but they require -
                                        1.   A Nuclear Localization Signal (NLS) on the protein.
                                        2.   Importin proteins (like Importin α and Importin β) to recognize the NLS and escort the protein through the nuclear pore.
Role of Importin β:
Importin β binds to the NPC and helps move the protein through it using energy (GTP via Ran-GTP cycle). It's the main carrier for large nuclear proteins.

Incorrect options -

Option a - Sec61  - It translocates proteins into ER and not involved in nuclear import
Option b - Tom20  - It helps in transporting the  proteins into mitochondria  and hence not related to nuclear envelope
Option d - Tim44 - It  helps in mitochondrial inner membrane protein import  and again, mitochondria not nucleus

So, the correct answer is option c - Importin B