"Selbstverständlich, Großvater."

Translation: Of course, grandfather.

[mystman1234]

Could you use "naturlich" here instead?

[christian]

Yes, but it's spelt "natürlich". It's not a good idea to just ignore the umlauts. There's always a difference in pronunciation and sometimes even a difference in meaning.

[wataya]

If you spend some additional money on the Umlaut, yes.

[jpentland]

Does this word kinda translate as "self-evidently"? Although it doesn't get used in this context in english much...