I picked Biology as the subjectthere is no subject classification listed for Geology. How long ago did the Molten surface of the earth cool to form the crust?

Around 4.54 billion years ago

Explanation:

The first rocky bits of Earth coalesced around 4.54 billion years ago, less than 100 million years after the solar system formed. For much of those early years, Earth was a blob of molten rock, but over time, the surface cooled and formed a crust that floated on the Earth's liquid core

characterization of nanoparticles is a critical step in nanomaterial production as it provides information of dimension, morphology, surface charge, distribution and degree of surface functionalization.