Answer:
At least 50% of the tetramers will be inactive for this individual
Step-by-step explanation:
A protein resulting from a mutation that is dominant negative will have an detrimental impact on the wild-type protein as well. Especially if the mutated protein can interact with the same substances as the normal, wild-type protein.
For an individual who is heterozygous, there will be one wild-type copy and one mutant copy of the allele present. For each copy, a protein will be expressed and the tetramers from the mutant copy will be defective - so 50% of the protein tetramers will be inactive at least.
Hope that answers the question, have a great day!