## Core Concept
Proteins that are encoded by nuclear DNA and destined for the mitochondria are synthesized as precursors in the cytosol and then transported into the mitochondria. The process by which these proteins cross the mitochondrial membranes is crucial for the functioning of the mitochondria.

## Why the Correct Answer is Right
The correct answer, **C. as a complex with chaperones**, highlights that many mitochondrial proteins are transported into the mitochondria as complexes with chaperones or other proteins that help guide them through the mitochondrial membranes. However, more specifically, proteins cross the outer mitochondrial membrane through a process involving **TOM (Translocase of Outer Mitochondrial membrane) complex** and then are often guided by **chaperones** or directly through **TIM (Translocase of Inner Mitochondrial membrane) complex** for inner membrane targeting. This process can involve various forms, but the concept of chaperones or specific complexes is essential.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because it does not accurately describe a recognized mechanism for protein transport into mitochondria.
- **Option B:** This option is incorrect as it suggests proteins cross in a free form, which underrepresents the complexity and the specific transport mechanisms involved.
- **Option D:** This option is incorrect because while proteins do interact with membranes, stating they cross "in a vesicular form" does not accurately represent the protein transport mechanisms into mitochondria.

## Clinical Pearl / High-Yield Fact
A key point to remember is that **mitochondrial proteins are encoded by both nuclear and mitochondrial DNA**, and their import is crucial for mitochondrial function. Defects in mitochondrial protein import can lead to various diseases, emphasizing the importance of this process.

## Correct Answer Line
**Correct Answer: C. as a complex with chaperones.**