Final answer:
Phylogeny is primarily determined using fossil records, genetic data, and behavioral observations, with fossil and genetic data being the most direct evidence. Hence, options (a), (b) and (c) are correct.
Step-by-step explanation:
To determine phylogeny, scientists primarily use fossil records, genetic data, and behavioral observations. Fossil records, including relative dating techniques, help determine the geological ages and the rate of speciation of early organisms, contributing to the understanding of their evolutionary history.
Genetic data, on the other hand, allows scientists to use DNA sequencing and computer programs to determine relatedness, providing a more precise method of constructing phylogenetic trees.
Behavioral observations can also provide insights into the evolutionary processes particularly when studying closely related species. All three types of data are important, but fossil and genetic data provide the most direct evidence for phylogenetics.