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Based on your requirement, you can use Update/Calculate conditions to automatically select the correct option for the "grade" field. Please follow the steps below:
1. Add an age field to your form.
2. Add the Update/Calculate condition. Here's a guide that can help: How to Insert Text or Mathematical Calculation into a Field Using Conditional Logic. Insert the answer that you would like to fill in the Grade field into the text area. So for example, if the Age is equal to 6, then the dropdown option will have 1º Primaria selected. Make sure that the text in the condition is exactly the same in your dropdown options. Below is a sample condition for Age 6.
Here's how it should work: