Which legislation or group would McElroy most likely support based on his letter?

The Pendleton Act of 1881 established a merit-based system for federal employment, significantly reducing political patronage and corruption in government hiring practices. Based on this context, McElroy's support for this legislation suggests that he valued professionalism and integrity within the government, aligning with the broader Progressive Era ideals of reforming political processes.

In contrast, the other options represent different aspects of governmental and economic regulations. The Sherman Antitrust Act was focused on curbing monopolies and promoting competition, which might not necessarily align with McElroy's concerns. The Interstate Commerce Act aimed to regulate railroads and ensure fair rates, addressing more economic issues than governance. The Federal Reserve Act established a central bank to manage monetary policy, which may not have been the primary focus of McElroy's advocacy for civil service reform. Therefore, the support of the Pendleton Act indicates a direct alignment with efforts to professionalize and reform the federal workforce.