[Plugin: QuizMe] How can we change options colour to black

The plugin leaves most of the css in control of the theme and as such you will not find css to change the colour of the options in the plugin. If your options are not already black then it but be the theme which is changing them.

Use a tool like Firebug (or my choice Chrome and inspect element right click) to review what CSS style (class) is being called and create a new style based on it that makes your site layout as desired.