Final answer:
The term that refers to all the characteristics observed during the lifetime of an organism is the phenotype. This describes observable traits such as hair color and height, resulting from the genotype's interaction with the environment. Options like genotype, allele, and behaviour represent different aspects of biological inheritance and traits but not observable characteristics.
Step-by-step explanation:
An organism's phenotype refers to all the characteristics observed during the lifetime of an individual. The correct answer to the question is option b) Phenotype. Phenotype is the set of observable characteristics or traits of an organism, which includes features such as colour, shape, and size, and these characteristics result from the interaction of the organism's genotype with the environment.
The genotype refers to the genetic makeup of an organism based on the DNA inherited from its parents, and it is represented by the combination of alleles for each gene that an organism has. While phenotype is what we can see, genotype is like the genetic blueprint that determines the potential for these traits. For example, Mendel's experiments with pea plants illustrated that plants with different genotypes (such as TT or Tt) could have the same phenotype (yellow pods) due to the dominance of one allele over the other.