The correct answer is option 'A' - Agmark.

Agmark is a certification mark in India that is used to ensure the quality and authenticity of agricultural products. It is a logo or mark that consumers can look for on a biscuit packet to ensure that the product meets certain quality standards. Here is an explanation of why the other options (b, c, and d) are not the correct answers:

b) ISI mark: ISI stands for Indian Standards Institute, which is a certification mark in India. However, the ISI mark is primarily used for certifying the quality and safety of industrial products, such as electrical appliances, pipes, and automotive components. It is not specifically related to biscuits or food products.

c) Hallmark: Hallmark is a certification mark used for precious metals, such as gold and silver. It is used to ensure the purity and authenticity of these metals, particularly in jewelry and other luxury items. It does not apply to biscuits or food products.

d) ISO mark: ISO stands for the International Organization for Standardization. ISO certification is a globally recognized standard for quality management systems. However, the ISO mark is not specifically used for individual products. It is more commonly seen on company websites or promotional materials to indicate that a company has been certified to meet certain international quality management standards.

Therefore, the correct answer for a biscuit packet would be the Agmark, as it specifically applies to agricultural products. This mark ensures that the biscuits have been certified for their quality and conformity to the standards set by the Agricultural and Processed Food Products Export Development Authority (APEDA) in India. By looking for the Agmark on a biscuit packet, consumers can have confidence in the quality and authenticity of the product.