Which of the following organisms can sense and respond to environmental cues?

The correct option is both (a) and (b).

All living organisms, regardless of whether they are unicellular or multicellular, simple or complex, possess the fundamental ability to sense changes in their surroundings and respond to these environmental cues. This characteristic is known as sensitivity or responsiveness.

Let's break down how both groups of organisms exhibit this ability:
1. Prokaryotes (Option b): These simple, single-celled organisms (such as bacteria and archaea) constantly monitor their environment. They can sense chemical gradients, temperature shifts, light, and magnetic fields, and respond through mechanisms like chemotaxis (moving toward nutrients or away from toxins) or by altering gene expression to adapt to stress.
2. Eukaryotes (Option a): These structurally complex organisms (including protists, fungi, plants, and animals) possess sophisticated sensory systems. For example, single-celled amoebae show chemotaxis, plants exhibit phototropism (growing toward light), and animals use dedicated sensory receptors and nervous systems to process and react to external stimuli.

Since both prokaryotes and eukaryotes can sense and respond to environmental cues, the correct choice is both (a) and (b).