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.
Twisted pair cabling is a type of wiring 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 neighboring pairs and improves rejection of external electromagnetic interference It was invented by Alexander Graham Bell