Phenetic classification is based on
(a) The ancestral lineage of existing organisms
(b) Observable characteristics of an existing organism
(c) Dendrograms based on DNA characteristics
(d) Sexual characteristics

Hint: The word phenetic has been derived from a Greek word phainein, which means ‘to appear.’ Phenetic classification of organisms is done based on measurable similarities and differences.

Complete step by step answer:
Phenetic classification is based on observable characteristics of an existing organism. The observable characteristics mean the morphology or other observable traits, regardless of their phylogeny or evolutionary relation.
- Taximetrics is the other name of phenetic classification.
- Phenetic classification is a part of numerical taxonomy, which is concerned with the use of numerical methods for taxonomic classification.
- Peter Sneath and Robert R. Sokal have a great contribution to developing such a method of classification.
- The word ``cladistics '' has been derived from the Greek word "branch". It is an approach to biological classification in which organisms are categorized in groups ("clades") based on the most recent common ancestor or ancestral lineage.
- A dendrogram is a diagram that represents a tree showing the correlation. In the field of computational biology, it shows the clustering of genes or samples.
- The word taxonomy is derived from a Greek word ‘taxis’, meaning 'arrangement', and ‘nomia’, meaning 'method.' It is the scientific study of naming, defining (circumscribing), and classifying groups of biological organisms based on shared characteristics.
- Carl Linnaeus developed a system known as Linnaean taxonomy for categorizing organisms and binomial nomenclature for naming organisms. He is regarded as the ‘father of taxonomy.’
So, the correct answer is, ‘(b) Observable characteristics of an existing organism.’

Note: Phenetic techniques include various forms of clustering (grouping in such a way that the organism in the same group is similar to those in other groups) and ordination (characterized by values on multiple variables so that similar objects are near each other and dissimilar objects are farther from each other.