Answer: d. μ<=80
Explanation:
Null hypothesis is statement written about the population parameter with respect to the objective of the test and against to the alternative hypothesis.
It always contains = , ≤ , ≥ signs.
Given : A researcher is conducting an experiment to evaluate a treatment that is expected to increase the scores for individuals in a population that is known to have a mean of μ = 80.
The results will be examined using a one-tailed hypothesis test.
Alternative hypothesis must be :

Then, null hypothesis will be :
[opposite to the alternative hypothesis.]
Hence, the correct answer is . d. μ<=80