One negative consequence of using automation to improve manufacturing production is that

(i) production will often slow down.

(ii) costs often rise for producers.

(iii) prices often increase for consumers.

(iv) factories often need fewer workers.

Answer

(iv) factories often need fewer workers.

Explanation

One negative consequence of using automation in manufacturing is the reduction in the need for human labor. As production processes become automated, machines and robots take over tasks that were previously performed by workers. This can lead to job displacement, as factories may require fewer employees to operate the same or even increased levels of production. While automation can certainly enhance efficiency and output, it can also lead to a reduced workforce, which is a significant negative consequence for the labor market and the individuals affected by layoffs.

The other options mentioned (i, ii, iii) focus on effects that are not as directly associated with the automation of labor in manufacturing, but rather pertain to broader economic impacts such as costs and pricing.