Under the Clayton Act, horizontal mergers by stock acquisition were a. not considered. b. illegal if they could be shown to lessen competition. c. illegal under any circumstances. d. legal if they could be shown not to lessen competition.

The Clayton Act, enacted in 1914, aims to regulate and maintain fair competition within industries. It specifically targets anti-competitive practices that cannot be addressed by the Sherman Antitrust Act. The Clayton Antitrust Act establishes a framework for regulating horizontal mergers (combinations of companies that compete in the same market) by stock acquisition. They are addressed in Section 7 of the Clayton Act.

Let's evaluate the given options: a. not considered: This option is incorrect. The Clayton Act aims to regulate business practices to maintain fair competition, and horizontal mergers by stock acquisition are considered as they are addressed in Section 7 of the Clayton Act. b. illegal if they could be shown to lessen competition: This option is correct. Under the Clayton Act, horizontal mergers by stock acquisition are illegal if they can be shown to significantly reduce competition or tend to create a monopoly. This provision aims to maintain a competitive market and protect consumers from the negative effects of monopolies. c. illegal under any circumstances: This option is incorrect. The Act does not make horizontal mergers by stock acquisition illegal under all circumstances; rather, it makes them illegal only when they can be shown to significantly reduce competition or tend to create a monopoly. d. legal if they could be shown not to lessen competition: This option can be considered misleading. Although horizontal mergers can be allowed if they do not lessen competition, in practice, the primary focus of the Clayton Act is to prevent mergers that would significantly reduce competition. Therefore, this option is not entirely accurate for explaining the provisions of the Clayton Act for horizontal mergers by stock acquisition Based on the analysis, option b is the most accurate representation of the Clayton Act's provisions on horizontal mergers by stock acquisition.

The correct answer is: b. illegal if they could be shown to lessen competition.