X is present in all eukaryotic cells except mammalian RBC, sieve tube cells of phloem and tracheids and vessels of xylem. X is _________ .

Options

A. Nucleus
B. Endoplasmic reticulum
C. Mitochondrion
D. Chloroplast

Answer

To solve the question, "X is present in all eukaryotic cells except mammalian RBC, sieve tube cells of phloem, and tracheids and vessels of xylem. X is _________," we can follow these steps:

### Step 1: Understand the Components of Eukaryotic Cells
Eukaryotic cells are characterized by the presence of a nucleus and membrane-bound organelles. Examples of eukaryotic cells include plant cells, animal cells, and fungal cells.

**Hint:** Recall the definition of eukaryotic cells and their main features.

### Step 2: Identify the Exceptions
The question specifies that X is absent in:
- Mammalian Red Blood Cells (RBC)
- Sieve tube cells of phloem
- Tracheids and vessels of xylem

This indicates that X is something typically found in eukaryotic cells but is absent in these specific cell types.

**Hint:** Think about the functions and structures of the cells mentioned and what they might lack.

### Step 3: Analyze the Options
The options provided are:
1. Nucleus
2. Endoplasmic reticulum
3. Mitochondria
4. Chloroplast

We need to determine which of these is absent in the specified cell types.

**Hint:** Consider the roles of each organelle and whether they are present in the specified cell types.

### Step 4: Evaluate Each Option
- **Nucleus:** Present in most eukaryotic cells but absent in mammalian RBC (which lose their nucleus as they mature), sieve tube cells, and tracheids/vessels.
- **Endoplasmic Reticulum:** Present in all eukaryotic cells.
- **Mitochondria:** Present in all eukaryotic cells.
- **Chloroplast:** Present only in plant cells, but not relevant to RBC.

**Hint:** Focus on the organelle that is specifically known to be absent in the mentioned cell types.

### Step 5: Determine the Correct Answer
Given that the nucleus is the only organelle that is absent in mammalian RBC, sieve tube cells, and tracheids/vessels, we conclude that:

**X is the nucleus.**

### Final Answer
X is **nucleus**.

---