Captcha to stop spam - form created and works

We have a form which is 3 pages long; you have to press next to view the following page of the form. The form works and is sent to two different email addresses with different information. The fields that are necessary are marked with a red start they validated and work. The form is brilliant, but we get spam, so we would like a captcha to be placed at the end of the form, just before the text ‘That’s it!’. Some captcha’s don’t work; we need this to work and to stop spam. I have attached 3 image of the different stages of the form, and also the for PHP code. We are working on a budget and for some one that knows what they are doing; I do not feel this is a big job at all. If possible we would like this done today. Thanks