Which of the following is a special defense against insider dealing?

Extensive market information serves as a special defense against insider dealing because it pertains to the possession of information that is generally available to the public, thus not falling within the remit of insider trading regulations. In the context of UK regulation, insider dealing involves trading based on confidential, price-sensitive information that is not publicly available. If an investor can demonstrate that their trading decisions were based on market information that is accessible to all, rather than exclusively on privileged insights, they are less likely to be found in violation of insider trading laws.

The other options—stock manipulation, anonymous trading, and unregulated speculation—do not provide valid defenses against insider dealing. Stock manipulation involves the alteration of a stock's price or trading volume, which is illegal irrespective of whether the trader has insider information. Anonymous trading does not inherently absolve an individual from the repercussions of insider trading; it simply hides the identity of the trader, which has no bearing on the legality of the information used. Unregulated speculation can imply risky trading practices without adherence to regulation, but it does not constitute a lawful defense against insider trading accusations. Thus, extensive market information is the only correct answer in the context of lawful trading practices in the UK.