**Core Concept**
The relative risk (RR) is a measure of the ratio of the probability of an event occurring in the exposed group versus the non-exposed group. In epidemiological studies, RR is often used to quantify the strength of association between an exposure (in this case, passive smoking) and an outcome (depression).

**Why the Correct Answer is Right**
A relative risk of 1.05 suggests that passive smoking is associated with a small increase in the risk of developing depression. This means that individuals exposed to passive smoking have a 5% higher risk of developing depression compared to those not exposed. However, this association is relatively weak, and the actual risk difference may be small. It's essential to consider the baseline risk of depression in the study population and the study's sample size to put this finding into context.

**Why Each Wrong Option is Incorrect**
**Option A:** A relative risk of 1.05 does not indicate a significant association between passive smoking and depression, as the RR is close to 1. However, this option is incorrect because it implies that there is no association, which is not supported by the data.

**Option B:** A relative risk of 1.05 does not indicate a protective effect of passive smoking against depression. This option is incorrect because it suggests that passive smoking would decrease the risk of depression, which is not supported by the study's findings.

**Option C:** A relative risk of 1.05 does not indicate a strong association between passive smoking and depression. This option is incorrect because it implies that the association is significant, which is not necessarily the case given the small RR value.

**Option D:** A relative risk of 1.05 does not indicate a significant increase in the risk of depression due to passive smoking. This option is incorrect because it suggests that the association is strong, which is not supported by the study's findings.

**Clinical Pearl / High-Yield Fact**
When interpreting relative risks, it's essential to consider the baseline risk of the outcome in the study population and the study's sample size. A small relative risk may not be clinically significant if the baseline risk is low, and a large relative risk may be statistically significant but clinically insignificant if the outcome is rare.

**Correct Answer: B.**