Plastids are present in

The correct answer is Both (a) and (b).

To understand why this option is correct, let us analyze the presence of plastids in different groups of organisms step-by-step:

1. What are Plastids?
Plastids are double-membrane-bound organelles found in the cells of plants and diverse eukaryotic algae. They are primarily responsible for activities related to making and storing food, containing pigments used in photosynthesis and various types of nutrients. The most well-known type of plastid is the chloroplast.

2. Presence in Plants (Option b):
All plants (kingdom Plantae) possess plastids. In plant cells, chloroplasts perform photosynthesis, chromoplasts store carotenoid pigments that give flowers and fruits their bright colors, and amyloplasts/elaioplasts/aleuroplasts (collectively leucoplasts) store starch, lipids, and proteins.

3. Presence in Euglenoids (Option a):
Euglenoids (like Euglena) are a group of unicellular protists. Most photosynthetic euglenoids contain plastids (chloroplasts) that allow them to perform photosynthesis in the presence of sunlight. These chloroplasts are surrounded by three membranes and contain chlorophyll a and b, similar to those found in plants.

4. Why not "All Protists" (Option c)?
Protists are a highly diverse kingdom. While some protists (like euglenoids, diatoms, and dinoflagellates) possess plastids, many other protists, such as amoeba, paramecia, and slime molds, do not have plastids. Therefore, plastids are not present in all protists.

Consequently, since plastids are present in both euglenoids and plants, the correct choice is Both (a) and (b).