Why is "mi abuelo es argentina" incorrect?

'Mi abuelo es argentino', is the right answer.

Explanation:

Mi abuelo es argentina is incorrect because abuelo means grandfather, who is obviously a male, and the adjective argentina is used to denote feminine gender because it ends with -a. Instead, if you want to correct this sentence, you should write the adjective in masculine form (ending with -o).

So, it should go like this:

Mi abuelo es argentino. 

same as the person below me