Answer: a. Remain an Oligopoly.
Explanation:
An Oligopolistic Market is one where there are few sellers controlling most of the industry.
Even if the anti-trust laws are strictly enforced, it still only remain a Oligopolistic industry.
Why?
The Only companies that will be affected are A and B.
A will split to 2 companies and so will B.
What will then happen is that 90% of the market is still controlled by 5 firms being A1 and A2 at 20% each. B1 and B2 at 15% each and C at 20%.
These 5 firms could still control price so this market is Oligopolistic.