How do i make a button click in one form change an option in another form? C#

Please don't!
That's the simple answer. You can't access the buttons on a differnt form because they are declared private by default. If they weren't, then the two forms are always tied together - if you change one, you have to look at how that change might affect the other form. This makes re-use difficult, and can affect reliability.

Instead, provide a public property or method in the second form, and access them from the first. The property or method makes changes to the form controls, so you can change the controls, without affecting the outside world.

All that leaves is for you to interact with the correct instance of the second form!
When you displayed, it you did something along the lines of:

MySecondForm secondForm = new MySecondForm();
secondForm.Show();

All you have to do is move secondForm to be a private class level variable, and you can access it when you need to:

secondForm.DisablePrinting();

or

secondForm.CanPrint = false;

Store the values in the registry. When the options form loads parse registry entries that dictate control state.. eg, dword reg entry chk1 = 0, checkbox loads unchecked, = 1, loads checked.
Another method is using the applications settings page, create a bool value, then access it through the forms.
bool ButtonVisible setting
this.btnOption.Visibile = Properties.Settings.Default.ButtonVisible;
Note you have to call: Properties.Settings.Default.Save(); after changing a value.

Here's a more complete example, including using the PropertyChanged event on the main form to propogate the change..

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
    }
    private void Form1_Load(object sender, EventArgs e)
    {
        //Properties.Settings.Default.ButtonVisible = true;
        Properties.Settings.Default.PropertyChanged += new PropertyChangedEventHandler(Default_PropertyChanged);
    }
    void Default_PropertyChanged(object sender, PropertyChangedEventArgs e)
    {
        button1.Visible = Properties.Settings.Default.ButtonVisible;
        this.Refresh();
    }
    private void button2_Click(object sender, EventArgs e)
    {
        frmOptions f = new frmOptions();
        f.Show(this);
    }
}

options form:

<pre lang="cs">
    public partial class frmOptions : Form
    {
        public frmOptions()
        {
            InitializeComponent();
        }
        private void frmOptions_Load(object sender, EventArgs e)
        {
            Properties.Settings.Default.ButtonVisible = false;
            Properties.Settings.Default.Save();
        }
    }

You can use session variables to store the values and use it in the other form. Take a look at this article [^] on msdn.

Ignore, saw web browser and took it to be asp.net project.

First of all, I agree with Griff on "don't"!

For the form collaboration, please see my universally applicable method based on implementation of an interface by the form class:
How to copy all the items between listboxes in two forms[^]; please also see other methods and discussion.

—SA