Final answer:
The scholar who first recognized the value of a speaker's credibility was Aristotle. His work on the concept of ethos is a key element in the field of rhetoric. Other notable figures include Marconi with the invention of the radio, Maslow for his hierarchy of needs, and Marx for his views on class struggle.
Step-by-step explanation:
The communication scholar who first recognized the value of a speaker's credibility was b) Aristotle. Aristotle's work on rhetoric covers the concept of ethos, which is one of the three modes of persuasion. Ethos refers to the character and credibility of the speaker, which Aristotle identified as crucial for persuasion. This concept is foundational in the field of communication and rhetoric.
In other historical contributions to knowledge, the invention of the radio is attributed to c) Guglielmo Marconi, who was a pioneer in the development of wireless communication. Also of note, the hierarchy of needs was proposed by c) Abraham Maslow, an influential figure in psychology. Lastly, the belief that the history of society was one of class struggle is famously associated with b) Karl Marx, who co-authored 'The Communist Manifesto' and where he elaborated on his theory of class struggles.