Final answer:
The 20-sided polyhedral capsid of an adenovirus is called an icosahedron, identified as answer (d). This shape contributes to the stability and robustness of the virus.
Step-by-step explanation:
The virus you are referring to, the adenovirus, which causes respiratory infections in mammals, is built with a capsid that has a specific three-dimensional shape. This shape is known as an icosahedron, which is a polyhedral shape composed of 20 equilateral triangles. This means the correct answer to the question is (d) icosahedron.
It's important to note that viruses can have different types of capsids, which include not only the icosahedral form but also helical, complex, and enveloped structures. The icosahedral shape is particularly common among viruses because, yet efficient structure, using repeated protein subunits to form its capsid. The equilateral triangular faces provide viruses like adenovirus a robust outer layer to protect their genetic material.