Many modern architects insist on_____ materials native to the region that will blend into the surrounding landscape.

Insist on doing something
- It is a Phrasal verb. 
- It means, to keep doing something, even if it annoys other people, or people think it is not good for you.

Ex: - I don't know why you insist on talking about it.
- They insist on playing their music late at night.

অতএব দেখা যাচ্ছে,,  insist on এর পরের main verb, ing যুক্ত হয় বা Present Participle form হয়। 
সুতরাং, সঠিক উত্তর হবে: using