Which 1914 law prohibited corporations from acquiring the stock of another corporation if doing so would create a monopoly?

The law you are referring to is the Clayton Antitrust Act of 1914. This legislation aimed to prevent anti-competitive practices by prohibiting corporate acquisitions that would substantially lessen competition or create a monopoly. It strengthened the earlier Sherman Antitrust Act and provided more specific guidelines on anti-competitive behaviors. The Clayton Act also addressed issues related to price discrimination and exclusive contracts.