Identify Marked Parts: Analyzing Red-Shaded Rectangles in a Grid Pattern

To determine the marked part, we need to calculate the fraction of the diagram that is shaded red.

First, we count the total number of rectangles in the diagram. There are 10 rectangles visible along a straight line.

Next, we count the number of rectangles shaded red. There are 8 red rectangles in the diagram.

Therefore, the fraction of the total diagram that is marked red is calculated as $\frac{\text{Number of Red Rectangles}}{\text{Total Number of Rectangles}} = \frac{8}{10}$.

This fraction simplifies to $\frac{4}{5}$, but the answer provided is in the form $\frac{8}{10}$, which is equivalent.

Therefore, the marked part of the diagram is $\frac{8}{10}$.