1.
seven hundred (700)
The number 700. 七 + 百, with no sound change.
七 + 百. Both parts keep their normal shape.
Pronunciation
The alternative しちひゃく is possible but rarely used; なな is preferred throughout the hundreds because しち is easily misheard as いち. The same preference shows up in 七時 versus 七人／七人, where speakers often choose なな for clarity.
Only 三百, 六百, and 八百 change; 七百 and the rest are regular.