If we separate the cell organelles of a living cell, then which part should be alive

The correct option is chloroplast.

To understand why a chloroplast can remain functional or "alive" when isolated from a living cell, we need to look at its structural and genetic characteristics:
1. Semi-autonomous Nature: Chloroplasts (along with mitochondria) are semi-autonomous organelles. This means they possess their own genetic material (DNA), as well as ribosomes (specifically 70S ribosomes) and the necessary machinery to synthesize some of their own proteins.
2. Metabolic Independence: Because they have their own DNA, RNA, and protein-synthesizing machinery, they can perform key life-sustaining activities such as transcription, translation, and energy conversion (photosynthesis) independently for a limited time when placed in a suitable nutrient medium.
3. Comparison with other options:
- Ribosomes and the endoplasmic reticulum are not self-replicating and do not contain their own genome; they cannot function or survive independently outside the cytoplasmic environment.
- The cell wall is a non-living, rigid outer layer composed primarily of cellulose and does not exhibit metabolic activity or life processes.

Therefore, among the given organelles, the chloroplast is the one that contains the autonomous machinery required to perform metabolic activities independently, allowing it to remain temporarily alive or functional when separated from the cell.