What design element of an astronaut's spacesuits protects him or her from micrometeoroids, which are tiny bits of space debris travelling at high speeds?

Tough outer shell

Explanation:

Three types of space suits based on their purpose are IVA (Intravehicular activity), EVA (extravehicular activity), and IEVA (intra/extravehicular activity)

Space suits have an outer layer known as the Integrated Thermal Micrometeoroid Garment also known as the TMG or ITMG

The functions of the TMG includes the provision of an insulation medium to the wearer of the suit thereby preventing loss of heat and keeping the wearer warm, protect the wearer from solar radiation which are harmful as well as from Micrometeoroid and debris orbiting in space known as MMOD

The outermost layer of the EMU TMG suits is white Ortho-Fabric that consists of Nomex, Kevlar and Gore-Tex, making it fire resistant, very strong and very tough, and water proof as well as breathable.