The valid filter declarations are filter: opacity(25%) and filter: drop-shadow(16px 16px 16px red). The CSS filter property applies graphical effects to an element, commonly including image rendering effects such as blur, contrast, color shifts, transparency, and shadows. opacity(25%) is a valid filter function because filter opacity accepts a percentage or number to control the rendered transparency of the element. drop-shadow(16px 16px 16px red) is also valid because drop-shadow() is a CSS filter function, and MDN shows that it accepts two or three length values plus an optional color. box-shadow(...) is invalid in this context because box-shadow is a standalone CSS property, not a filter function; MDN explicitly distinguishes drop-shadow() from the box-shadow property. blur(25deg) is invalid because blur uses a length value such as pixels, not an angular value such as degrees. References/topics: CSS filter property, image effects, opacity filter, drop-shadow filter, valid filter-function syntax.

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