A) Mild steel
B) Cast iron
C) Aluminum
D) Copper

Explanation:

As per IS 1608, when a ductile material like mild steel is subjected to tensile test, it undergoes significant plastic deformation before failure. This causes necking at the fractured section, where the diameter reduces substantially.

However, brittle materials like cast iron and aluminum do not undergo plastic deformation. In their case, the diameter at fracture remains same as original diameter.

Since the given specimen did not show reduction in diameter at fracture, it indicates a brittle failure without plastic deformation. Therefore, the material was most likely cast iron.

Reference:
IS 1608:2005, Metallic Materials – Tensile Testing at Ambient Temperature

The correct option is B.