Question

How did Theodore Roosevelt regulate the growth of trusts?

Answer 1

Answer:  He enforced the Sherman Antitrust Act.

Context/history:

The Sherman Anti-Trust Act  was the first measure by Congress to prohibit trusts. It was passed by Congress in 1890.  A trust was when  stockholders in multiple  companies transferred their stock shares to a single group of trustees. Thus a whole industry area could be dominated by a single "trust" organization, destroying the free market of business competition. This was a monopolistic practice which the Sherman Anti-Trust Act ended.  Thus the Sherman Anti-Trust Act directly went against the idea of those who believed business success should be based on large business owners colluding with one another.

Initially the Sherman Antitrust Act was not well enforced by US courts.  But when Theodore ("Teddy") Roosevelt took office as President in 1901, he pushed enforcement of the Act and worked to reign in the power of big businesses.

Note:  

The Clayton Antitrust Act was passed by Congress in 1914, after Teddy Roosevelt was no longer President.