The brown fumes evolved upon heating lead nitrate are primarily composed of:

The correct option is Nitrogen dioxide.

Explanation

The phenomenon described is a thermal decomposition reaction, where a single compound breaks down into multiple simpler substances upon the application of heat.

Reaction Analysis:

• When lead nitrate powder [$Pb(NO_3)_2$] is heated strongly in a boiling tube, it decomposes to form lead oxide ($PbO$), nitrogen dioxide ($NO_2$), and oxygen ($O_2$).
• The balanced chemical equation for this reaction is:
  $2Pb(NO_3)_2(s) \xrightarrow{\text{Heat}} 2PbO(s) + 4NO_2(g) + O_2(g)$
• Visual Observation: The emission of reddish-brown fumes is the specific characteristic of Nitrogen Dioxide ($NO_2$) gas.
• The residue left behind is Lead Oxide ($PbO$), which is yellow in color, while Oxygen ($O_2$) is a colorless and odorless gas.

Key Takeaway:

The thermal decomposition of Group 2 and heavy metal nitrates typically results in the formation of a metal oxide, oxygen, and nitrogen dioxide, the latter being identifiable by its distinct brown fumes.