The differences appear to be the addition of comments to certain lines of the program. This is called a patch, and there are programs to apply one. This one looks like it was generated with Git. You can see the "context", introduced by "@@" lines surrounding the change, and the lines prefixed by "-", which are to be removed, and the lines prefixed by "+", which means they are added.

The code should transform from:

void phonep()
{
    int pres;
 
    do
    {
        pres = (rand() % 10) + 1;
    } while(answers[qnum][pres - 1] == "");
 
    if(pres < 5)
    {
        // Stuff ...
    }
    // More stuff ...
}       

To:

void phonep()
{
    int pres;
 
//    do
//    {
        pres = (rand() % 10) + 1;
//    } while(answers[qnum][pres - 1] == "");
 
    if(pres < 5)
    {
        // Stuff ...
    }
    // More stuff ...
}