What is the word that scientists use to describe an organism's size, shape, and composition, in light of natural selection pressures?

The correct answer is - Morphology

Explanation:

Scientists use the term morphology to describe the size, shape, and composition of an organism in light of natural selection pressure. Some scientists used morphology to classify the organism.

The organism which comes in the same taxa are more morphological similar to each other than the organism who are present at different taxa. Morphology plays an important role in any species fitness.

Morphological traits evolve over time due to natural selection pressure and provide variation in offspring morphology during evolution. So the correct answer is morphology.