a) Viruses are usually units consisting of nucleic acids and coat proteins called capsids.
Viroids are small, circular, single-stranded molecules of infectious RNA that cause several plant diseases.
Prions are infectious protein particles responsible for a group of transmissible and/or inherited neuron-degenerative diseases as a result of prion protein mis-folding.
b) A bacteriophage, also known informally as a phage, is a virus that infects and replicates within bacteria and archaea. The term was derived from "bacteria" and the Greek φαγεῖν, meaning "to devour".
c) The shapes of viruses are classified into four groups: filamentous, isometric (or icosahedral), enveloped, and head and tail. Filamentous viruses are long and cylindrical.
d) A lysogenic virus can remain inactive for a period of time. In lysogenic infection, viral DNA gets integrated with the host cell's DNA, where it is copied along with the host cell's DNA when the host cell replicates.