A Horizontal Merger is so called if :

a. Companies in the same industry and same level of supply chain merge.

Horizontal integration is intended by the merger between the two institutions are working in the same economic activity, such as the merger of the two institutions for the pharmaceutical industry or the two institutions for the production of cement .. and horizontal integration is to achieve economies of scale, and creates a monopoly powers of the merged company .. 

A is correct

The answer for this question is  "A"

I will select "A", and respect the answer of Mr. VENKITARAMAN KRISHNA MOORTHY VRINDAVAN.

Option a) Companies in the same industry and same level of supply chain merge. - is the correct answer.

The Answer A

a. companies in the same industry  and same level of supply merge + b. to a certain extend

A horizontal merger occurs when two competing businesses join together and form a new entity. An example would be Apple computers joining together with Microsoft computers and forming a new computer-based company called AppleSoft (or whatever). Issues arise as to whether the merger will constitute unfair competition or an unreasonable restraint on trade. Basically, there are guidelines in place to prevent monopolies from forming (except in certain areas).