The relationship that most likely reflects correlation but not causation is:
When the weather is snowy, people buy holiday gifts more often.
While there may be a correlation between snowy weather and increased holiday gift purchases, it is not necessarily the snowy weather that causes people to buy gifts. Rather, the holiday season itself (e.g., Christmas) is the primary factor driving gift purchasing, regardless of the weather.
In contrast, the other two relationships (sunny weather leading to more sunglasses use, and rainy weather leading to more umbrellas being carried) are more directly related to the immediate conditions influencing behavior, indicating a stronger causal link.