Final answer:
Calcium base greases offer excellent rust protection but are not suitable for high-speed applications due to their limited temperature range and tendency to stiffen.
Step-by-step explanation:
The semi-solid lubricant that provides good rust protection but is not suitable for high-speed applications is C) calcium base greases. Calcium base greases are known for their superior water resistance and excellent rust protection properties. However, they are not ideal for high-speed applications due to their limited temperature range and tendency to become stiff under such conditions, which can lead to inadequate lubrication and increased wear.