Answer: a. Amphibians and birds share a more recent common evolutionary ancestor than sharks and birds.
Explanation:
A cladogram is a diagrammatic representation of the phylogenetic relationship of recently evolved species with their common ancestors. The relationship is defined on the basis of genetic, molecular, morphological make up of the organisms. The node in the cladogram defines a divergence or difference between the two taxon.
Birds and amphibians share a more recent common ancestor than shark and birds because they can be seen lying closer at the terminal ends of the cladogram.