The correct consequence of the Gulf Stream becoming weaker is:
It will result in warmer temperatures in Europe.
A weaker Gulf Stream can lead to reduced heat transport to Europe, potentially resulting in cooler temperatures in the region, but it is often discussed that it could lead to more climatic anomalies overall. However, in terms of direct consequences of a weaker current impacting temperature, the statement reflects that the heat distribution in the Atlantic may be altered, potentially leading to different temperature outcomes in Europe.