Which of the following is NOT a valid CSS property?*2 pointsfont-sizebackground-colortext-colormargin-top

All of the options provided are valid CSS properties. These include:

1. font-size: This property is used to specify the size of the font. Values can be specified in length units or as a relative size.

2. background-color: This property sets the background color of an element. It can take color names, hex color codes, RGB, RGBA, HSL, HSLA values.

3. text-color: This is not a valid CSS property. The correct property to set the color of the text is 'color'.

4. margin-top: This property sets the top margin of an element. It can take values in length units, percentages, or the keyword 'auto'.

So, the answer is 'text-color' as it is not a valid CSS property.