Which holds more a CD a DVD or a usb flash drive?

CDs hold around 700-800Mb,

DVDs hold around 4.5Gb (or 9Gb for dual-layer)

Flash drives can come in 128Mb, 256Mb, 512Mb, 1Gb, 2Gb, 4Gb, 8Gb, 16Gb and more recently, 32Gb.

Flash drives can hold more.

Blu-rays can hold up to 200Gb (although only 50Gb or 100Gb are readable with standard equipment).