STP (Shielded Twisted Pair) is a type of Ethernet that has an EMI resistant jacket. Because there are a large amount servers, STP is necessary. UTP should be avoided in data centers because they are susceptible to crosstalk. Fiber is incorrect because the question specifies we need a cost effective solutions. Typically, fiber will be used on trunk and backbone connections.
Wikipedia
Twisted pair cabling is a type of communications cable in which two conductors of a single circuit are twisted together for the purposes of improving electromagnetic compatibility. Compared to a single conductor or an untwisted balanced pair, a twisted pair reduces electromagnetic radiation from the pair and crosstalk between neighbouring pairs and improves rejection of external electromagnetic interference. It was invented by Alexander Graham Bell.For additional noise immunity, twisted-pair cabling may be shielded. Cable with shielding is known as shielded twisted pair (STP) and without as unshielded twisted pair (UTP).