The correct answer is - a. large membership.
Al-Qaeda was the biggest terrorist organization until recently. It was financially strong, had numerous basis and training camps, had solid amount of weaponry, and it had affiliations in lot of Muslim countries. The biggest problem for the US forces though was the large membership of al-Qaeda. They had millions of members. While some were engaged directly on the battlefields, lot of them were all over the place, acting like normal civilians, and they were the ones that made the biggest problems. Because there were so many members, and they were dispersed over a very large territory, it needed quite some time that the US forces managed to defeat and dismember the organization, with lot of casualties on the way.
