WASHINGTON — Inflation in the United States dropped last month to its lowest point since it first began surging more than three years ago, adding to a spate of encouraging economic news in the closing weeks of the presidential race. Consumer prices rose just 2.4% in September from a year earlier, down from 2.5% in August, and the smallest annual rise since February 2021. Measured from month to month, prices increased 0.2% from August to September, the Labor Department reported Thursday, the same as in the previous month. But excluding volatile food and energy costs, “core” prices, a gauge of underlying inflation, remained elevated in September. Core prices in September were up 3.3% from a year earlier and 0.3% from August. …The improving inflation picture follows a mostly healthy jobs report released last week, which showed that hiring accelerated in September and that the unemployment rate dropped from 4.2% to 4.1%.