Final answer:
The decision rule for the given hypothesis test is to reject H₀ if z > 1.96 or z < -1.96.
Step-by-step explanation:
To determine the decision rule for this hypothesis test, we need to find the critical z-values for a significance level of 0.05. Since it is a two-tailed test (H₁: μ ≠ 590), we divide the significance level by 2 to get 0.025 for each tail. Looking up this value in the standard normal distribution table, we find that the critical z-values are approximately -1.96 and 1.96. Therefore, the correct decision rule is a) Reject H₀ if z > 1.96 or z < -1.96.