The correct answer is B. Eldest / youngest.



1. Context of the Sentence:
- The sentence is comparing the age positions within a family. "Eldest" refers to the oldest in the family, while "youngest" refers to the one who is the least old.

2. Word Meanings:
- Eldest: This term refers to the person who is the oldest among siblings.
- Youngest: This term refers to the person who is the least old among siblings.

3. Usage in the Sentence:
- "Eldest" is used to describe the oldest sibling.
- "Youngest" is used to describe the least old sibling.

So, the sentence correctly becomes: "While I am the eldest child in the family, John is the youngest."