Final answer:
John Stuart Mill identified social tyranny from fellow citizens as the main threat to free speech, where societal pressure forces individuals to conform and suppresses minority opinions.
Step-by-step explanation:
The main threat to "free speech" according to John Stuart Mill was a) Social tyranny from fellow citizens. In his work On Liberty, Mill cautioned against society's potential to impose its views and silencing those who dissent, irrespective of whether those views were supported or enforced by the government. This form of social pressure can result in a tyranny of the majority, where minority opinions are suppressed out of fear of social backlash or ostracism.
Mill's commitment to individual liberty was rooted in his belief in utilitarianism, but with a particular emphasis on the importance of personal freedom and expression. The concept of individual rights, including the right to free speech, must be protected against all potential suppressors, including the state, but importantly also against the narrower interests of the society or the majority that could impose a social tyranny. According to Mill, true liberty meant not only the absence of governmental restraint but also freedom from societal pressures that compel individuals to conform.