How to 'mark' correct/incorrect text selection

Hi, I am trying to write a simple piece of code for an experiment which shows a bitmap in the centre of the screen (using show bitmap command) and then shows four words, one in each corner (using the text, plus stimulus position commands). 
The participant then selects one of the words and I need to feedback whether they chose the correct word and total up their 'corrects' to present to them at the end of a number of trials. 

I am struggling to find the syntax to mark their selection as correct/incorrect and save their score. Would you be able to help please?

Thanks