Explanation:
First blank: img
Second blank: filter:
Third blank: grayscale(100%)
Fourth blank: blur(8px)
The correct CSS selector is img because the requirement applies the visual effect to gallery images. The correct property is filter:, not transform:, because CSS filters modify the rendered appearance of an element using image-processing effects such as blur, grayscale, opacity, brightness, contrast, and shadows. The filtered sample appears blurred and colorless, so the appropriate filter functions are grayscale(100%) and blur (8px). grayscale(100%) removes all color from the image, converting it to a black-and-white rendering. blur (8px) obscures visual details, making the image difficult to recognize until the protected or authorized state changes the styling. The filter keyword without a colon is not a valid CSS declaration in this context.
brightness(40%) and opacity(50%) could further obscure an image, but they are not required to reproduce the shown filtered result. The final declaration is therefore filter: grayscale(100%) blur(8px);.