Cells transport molecules into the cell in many different ways. one method is through endocytosis, shown in the model. in endocytosis, the cell brings specific molecules it needs, including fluid, into the cell. why does the structure of the plasma membrane make this type of transport necessary for fluids?

Plasma membrane is the structure which covers up the uptaken molecule.

Uptaking of nutrients from the surrounding needs to be very appropriate for the cell to obtain nutrition, as well as to maintain cell immunity. In case of simpler molecules like glucose etc, the molecules are taken inside the cell by means of specific channels or transport proteins. In case of complex molecules, cells prefer to perform endocytosis - either phagocytosis for solid ones, and pinocytosis for liquid ones.

When the cell finds something which can be taken in, it projects a fold of cell membrane towards the liquid which surrounds part of the liquid, or fully. This cell membrane then presents a depression which covers up the liquid. Then the margins of the depression come closer and ultimately fuse, leaving the liquid inside the cell enclosed with a part of cell membrane. This then fuses with a primary lysozome to form a digestive vacoule, a secondary lysozome, a residual body, and ultimately gets excreted off the cell.

it cools slower in the earth meaning the crystals have more time to form than lava on the surface

since there are no options posted i'll give you a few examples.

a dolphin's flipper, bird's wing, cat's leg.