The correct answer is option 'A': Lost wax.

The technique of lost wax casting was widely used by the Harappans for bronze casting. This technique is also known as investment casting or cire perdue. It is a method of creating a metal object by pouring molten metal into a mold that has been created by means of a wax model. Let's explore this technique in more detail:

1. Lost Wax Casting Process:
- Creation of a Wax Model: The first step in the lost wax casting process involves creating a wax model of the desired object. Skilled artisans would sculpt the model by hand or use molds to create intricate designs and details.
- Assembly: Once the wax model is ready, it is attached to a wax sprue, which is a wax rod that acts as a channel for the molten metal to flow into the mold.
- Investment: The wax model, along with the sprue, is then coated with a ceramic or plaster material known as the investment. This investment is made up of several layers to create a strong and heat-resistant mold.
- Burnout: The investment mold is heated to a high temperature in a kiln, causing the wax to melt and flow out, leaving behind a hollow cavity in the shape of the desired object.
- Bronze Casting: Once the mold is ready, it is filled with molten bronze or other metals. The metal is poured into the mold through the sprue and allowed to cool and solidify.
- Removal of Investment and Finishing: Once the metal has cooled, the investment is broken away to reveal the bronze object. The sprue is removed, and the object is cleaned, polished, and finished as required.

2. Advantages of Lost Wax Casting:
- Intricate Designs: Lost wax casting allows for the creation of highly detailed and complex designs that would be difficult to achieve through other casting methods.
- Reproducibility: The molds used in lost wax casting can be reused multiple times, making it possible to reproduce multiple copies of the same object.
- Dimensional Accuracy: The lost wax casting process ensures a high level of dimensional accuracy, resulting in precise and consistent reproductions.
- Versatility: Lost wax casting can be used to create a wide range of objects, from small jewelry pieces to large statues and sculptures.

In conclusion, the Harappans extensively used the lost wax casting technique for bronze casting due to its ability to create intricate designs and reproduce objects with high accuracy. This technique played a significant role in the development of their bronze artifacts, showcasing their advanced craftsmanship and artistic skills.