Question

scientists gave the mummy the nickname \ötzi\ (urt - see) because of where he had been found. they sent a tiny piece of ötzis hipbone to a lab in england. they used a special method to determine his age. they found ötzi was about 5,300 years old! he had lived during the stone age, when people still used stone tools to hunt.
small details around ötzi help tell a story about his life and times. powdery bits of pollen and scraps of plants found near his mummy revealed that ötzi had lived in what is now northern italy. today, ötzi lies in a special freezer in italy.
how did scientists learn that ötzi was thousands of years old?
a small part of ötzis was sent to a lab and to determine his age.

Explanation:

The text states that a tiny piece of Ötzi's hipbone was sent to a lab. In a lab, the usual process to determine age (like in archaeology/biology - related to Natural Science's Biology sub - field in a way as it's about organic matter analysis) is to study the sample. Freezing is not a method to determine age.

Answer:

A small part of Ötzi's hipbone was sent to a lab and studied to determine his age.