Answer:
d. environment alters phenotype.
Step-by-step explanation:
Phenotype refers to the physical characteristics of an organisms such as height, weight, color etc.
Genotype is the genetic composition of an organism.
The phenotype is usually as a result of the environment having an influence on the genotype.
In this case, the other twin is exposed to an environmental condition which is better nutrition. This better nutrition makes the twin taller. Tallness is a phenotypic trait.
This simply means environment altered the phenotype.