The industrial activity held least responsible for global warming is:

Among the given industrial activities, we need to identify the one least responsible for global warming.

Global warming is primarily caused by greenhouse gas emissions, especially CO₂.

Steel manufacturing: Produces large amounts of CO₂ from coke used in blast furnaces.

Electricity generation in thermal power plants: Burns fossil fuels (coal, gas), releasing enormous quantities of CO₂. This is one of the largest contributors to global warming.

Industrial production of urea: Uses CO₂ as a raw material ($$CO_2 + 2NH_3 \rightarrow (NH_2)_2CO + H_2O$$) but the process itself involves significant energy consumption and emissions.

Manufacturing of cement: While cement production does produce CO₂ (from limestone decomposition and fuel burning), its contribution is relatively lower compared to steel and thermal power generation in terms of the overall industrial share.

Therefore, among the given options, manufacturing of cement is held least responsible for global warming.