Hi Susie, any optic that will mount to a picatinny rail will work. If you want a red dot sight, I suggest this one budsgunshop.com/product_info.php/products_id/6997/sig+sauer+electro-optics+sor52001+romeo5+1x+20mm+obj+unlimited+eye+relief+2+moa ,
If you are looking for a low budget scope, look here budsgunshop.com/product_info.php/products_id/5007/truglo+tg8514bc+tru-brite+1-4x+24mm+obj+92.6-23.03+ft+@+100+yds+fov+30mm+tube+b ,
Stay safe & have a great day!

Hi Susie and welcome. Mostly any Red Dot, Scope or Flip Up Sights of your choice. this will depend on your intended use of the rifle. The AR is one of the most customizable platforms there is and there are hundreds of Optic options. Cheers and stay safe!
budsgunshop.com/product_info.php/products_id/104785/tacfire+is002+low+profile+sight+set+ar+platform+black+anodized+flip+up -------------------------------- budsgunshop.com/product_info.php/products_id/6997/sig+sauer+electro-optics+sor52001+romeo5+1x+20mm+obj+unlimited+eye+relief+2+moa ------------------------------- budsgunshop.com/product_info.php/products_id/104972/bushnell+ar731240+ar+optics+3-12x40mm+obj+29-7+ft+@+100+yds+fov+1+tube+black+matte+drop+zone+223

anything you want you just have to use a picatinny rail mount I like this for a inexpensive scope budsgunshop.com/product_info.php/products_id/90965/ncstar+stp3942gv2+markiii+3-9x42mm+il+p4+sniper+36.8-12+ft@1 and for a red dot I use this on my S&W M&P 15 budsgunshop.com/product_info.php/products_id/10139/ncstar+d4b+d4rgb+dual-illum+1x+24x34mm+obj+unltd+relief+3+mo