If you want to restrict the input of a cell in a spreadsheet, you can create dropdown menus using Data Validation.
To create a new validation rule, click Validation in the Data toolbar and choose Add New Rule to open the Data Validation panel.
Here, enter a name for the new rule and choose the applicable cell or cells if different than the current selection.
Next, you'll need to choose where the options for the dropdown will be sourced from. To use options from a range on the sheet, choose Range and select the range to reference. To create a list of options within the rule, choose List then enter the options that will appear in the dropdown.
Once you've finished setting up your new rule, click Save Rule to finish.
To manage existing rules, click Validation in the toolbar and choose Manage Rules to open the Data Validation panel. Here you can see all existing rules and modify those you have access to.
Highlight the rule you want to modify and open the action menu to the right of the rule name. You can then open the rule for editing, delete the rule, or apply the rule to your current selection.
To remove a rule or rules from a selection, highlight a range and click Validation in the toolbar. Choose Clear Selection to remove any rules from the chosen range. You can remove all validations from the sheet by choosing Clear Entire Sheet from the validation menu.