Answer:
Marco and his friends have to wait in a long line.
Step-by-step explanation:
A complication is any form of an obstacle of conflict that hinders the progress of work or aim. It may be in the form of an emotional barrier or even a physical barrier, as long as it prevents the character/ characters from achieving their goal or desired aim.
In the given scene, Marco and his friend decided to meet at the water slide. Once together, they went up the wooden stairs, which hardly pose a problem for them for they had been through it numerous times. But this time, the movement up the stairs was a task in itself not because of the sheer number of stairs it'll take them to reach the top, but because of the number of people in the crowd. Their efforts to get to the top were "slowed down by a surprising mid-day crowd ..... dozens of eager park patrons". Liam's statement that "they would need a little more patience than usual waiting for their favorite view from the top" reveals their progress is slow and will take them a while to navigate the long line to get to the top.
Thus, the complication here seems to be that Marco and his friends have to wait in a long line to get to their destination.