Final answer:
Colin Powell stated his 2003 UN speech would remain a blot on his record because the intelligence information he presented about Iraq's WMDs, which justified the invasion, turned out to be false.
Step-by-step explanation:
In a 2003 speech to the United Nations, Secretary of State Colin Powell presented what he claimed to be corroborated facts regarding Iraq's possession of weapons of mass destruction. However, by 2005, after the invasion of Iraq, it was widely recognized that these allegations were based on faulty intelligence. Powell himself acknowledged that the claims he made in the speech were false, and that presenting inaccurate information to the United Nations was a lasting stain on his record, leading to many questioning the justification for the Iraq War.
The correct answer to why Powell said in 2005 that his 2003 speech would always be a blot on his record is d. The intelligence information that he presented turned out to be false. This was a significant admission since the false intelligence played a crucial role in the decision by the United States and its allies to invade Iraq in 2003.