Which is correct? reason for why~ vs reason as to why~
Which is correct?

That's the reason for why I applied to this company.
That's the reason as to why I applied to this company.Is this also possible? That's the reason for I applied to this company.

Question

Which is correct? reason for why~ vs reason as to why~
Which is correct?

That's the reason for why I applied to this company.
That's the reason as to why I applied to this company.Is this also possible? That's the reason for I applied to this company.

Answer

"This / that is the reason why..." is correct.

Less formal - just "that's why"

Don't use contractions e.g. "that's" in formal writing.

Non perdere l'opportunità di imparare una lingua comodamente da casa tua. Esplora la nostra selezione di insegnanti di lingue esperti e iscriviti subito alla tua prima lezione!