Okay, let's tackle this question. The question is about a genetic material that can exist both chromosomally and extrachromosomally. Hmm, I need to recall which genetic elements have that dual capacity.

First, I remember that plasmids are extrachromosomal elements, but they don't integrate into the chromosome. Then there are things like transposons, which can move between locations. Oh wait, maybe it's a plasmid that can integrate, but no, plasmids usually stay separate.

Wait, there's also episomes. Episomes are genetic elements that can exist either as plasmids or integrate into the host chromosome. Oh right, like certain viral vectors or bacterial plasmids that can integrate into the host genome. For example, lambda phage in bacteria can exist as a plasmid or integrate into the bacterial chromosome. That makes them episomes.

So the answer is probably episome. Let me check the options. If the options include episome, that's the correct one. The other options might be things like plasmid, which is only extrachromosomal, or chromosomal DNA, which is only chromosomal. Mitochondrial DNA is extrachromosomal but doesn't integrate into the nucleus.

So the key here is the dual existence. Episomes can replicate as plasmids or integrate into the host chromosome. That fits the description. So the correct answer should be episome.

**Core Concept**
This question tests understanding of genetic elements capable of both chromosomal integration and extrachromosomal existence. The key concept is **episomes**, which are DNA molecules that can exist independently or integrate into host chromosomes. This is critical in viral replication (e.g., bacteriophages) and vectors in molecular biology.

**Why the Correct Answer is Right**
Episomes, such as the **lambda phage** in *E. coli*, can function as autonomous plasmids or integrate into the host genome via site-specific recombination. They maintain genetic information outside the host chromosome (extrachromosomal) and, when triggered, insert into the host DNA (chromosomal). This duality allows for both lytic and lysogenic life cycles in phages.

**Why Each Wrong Option is Incorrect**
**Option A:** *Plasmids* are strictly extrachromosomal and do not integrate into host chromosomes.
**Option B:** *Mitochondrial DNA* is extrachromosomal but never integrates into nuclear DNA.
**Option C:** *Chromosomal DNA* resides exclusively within the nucleus and does not exist extrachromosomally.

**Clinical Pearl / High-Yield Fact**
Remember **"Episome = episodic integration"**—think of episomes as "dual-life" genetic elements that can switch between plasmid and chromosomal states. This is a classic exam trap: confuse plasmids (extrachromosomal only) with episomes (dual capacity).

**Correct Answer: C. Episome**

✓ Correct Answer: B. Episome