Based on the options provided, the most reasonable conclusion about steel consumption and population size in 1850 is: "Steel consumption was around 100 million and the population was beginning to grow."
In 1850, global steel consumption was not in the billions; thus, options mentioning "100 billion" or "the population was around 100 billion" are inaccurate. The population was indeed increasing during this period, which aligns with the selected statement.