The correct answer is B) loading and discharging. The most frequent incidents of pollution during tanker operations occur during the loading and discharging of oil cargo. This is because these operations involve the transfer of large volumes of oil, and any spills or leaks during these activities can result in significant pollution. Regulations such as the Oil Pollution Act of 1990 and international conventions like MARPOL require strict procedures and safeguards to be in place during these critical operations to minimize the risk of spills. The other options are less likely to be the main cause of pollution incidents. Groundings (A) and collisions (D) can indeed lead to oil spills, but they are less common occurrences compared to the routine loading and discharging operations. Routine discharge of oil during ballasting and tank crude oil washing (C) is generally well-regulated and controlled to prevent significant pollution.