Which type of flashing is more difficult to install correctly when dealing with brick or stone walls?

Counter-flashing is indeed the correct answer because it is specifically designed to be integrated with masonry elements like brick or stone. The installation of counter-flashing requires that it interlocks with the masonry, which often involves cutting into the brick or stone to create a groove or recess. This adds complexity to the installation process, as precision is crucial to ensure that the flashing adequately redirects water away from the wall and prevents moisture infiltration into the structure.

In contrast, base-flashing, snow-flashing, and drip-flashing are generally simpler to install in combination with other materials. Base-flashing is positioned at the base of a wall to protect against water intrusion, while snow-flashing deals with snow accumulation on roofs. Drip-flashing, which directs dripping water away from critical areas, can often be installed in a more straightforward manner without the need for significant alterations to existing masonry. Thus, counter-flashing is indeed the most technically challenging to install correctly on brick or stone walls due to the required integration and precise fitting needed for effective water management.