Question

fill in the blank with the organelle that performs this function. the
choose your answer...
are the structures used to propel substances across the surface of a cell.

Explanation:

The question is about identifying the organelle (or cellular structure) that propels substances across a cell's surface. Cilia are hair - like structures on the cell surface that move in coordinated waves to move substances (like mucus in the respiratory tract) across the cell surface. Flagella are more for cell movement rather than moving substances across the cell surface. So the structure that fits is cilia.

Answer:

Cilia