Hi There!
Question - Which word best describes the hot, weak, inner part of the upper mantle, which can flow?
Answer - Asthenosphere
Why:
Lets check all the reasons in the question with the options.
Asthenosphere
Hot? Yes
Weak? Yes
Inner Part Of The upper Mantle? Yes
Flow? Yes
Lithosphere
Hot? Yes
Weak? No
Inner Part Of The upper Mantle? Yes
Flow? No
Crust
Hot? Yes
Weak? No
Inner Part Of The upper Mantle? Yes
Flow? No
Core
Hot? Yes
Weak? No
Inner Part Of The upper Mantle? I don't think so
Flow? No
Astheno sphere is the only choice that is all true in the question stated!
Hope This Helps :)