Answer: $800
Step-by-step explanation:
Even though your company is using only 75% of the space which means that the company is in effect only using 75% of the rental cost of $10,000, the remaining 25% will not count as opportunity cost because the rent of $10,000 is a fixed cost that is paid on ALL the space and so cannot be separated into costs per space.
The relevant opportunity cost is therefore the amount that the company can get if it decides to sub-let this excess space for $800 because this amount is an extra benefit to be lost if the opportunity is not taken.