The problem involves understanding the effect of changing wheel diameters on the number of revolutions made by the front wheels compared to the rear wheels of a tractor.

Initially, the front wheels make 2 revolutions for every 1 revolution of the rear wheels. This is reflected on the graph where the initial ratio is 2:1. The goal is to identify what change in wheel diameters brings the ratio towards 1:1, as shown on the graph.

From the graph provided, we see that as the diameters change, the ratio of front wheel revolutions per rear wheel revolution approaches 1.0. This indicates that at this point, both the wheels make the same number of total revolutions over a given distance.

Let's analyze the options:

• Only the rear wheel diameter is decreased: If only the rear wheel diameter is decreased, each rear wheel revolution will cover less distance, increasing the ratio of front to rear revolutions beyond 1:1. This is incorrect.
• Both the rear and front wheel diameters are decreased equally: Reducing both equally keeps the original ratio unchanged (2:1). This is incorrect.
• Both the rear and front wheel diameters are increased equally: If both diameters are increased equally, the revolution ratio would become closer to 1:1 as both wheels would cover more distance per revolution without changing the relative number of revolutions between them. This aligns with the observed shift in the graph towards 1:1, making it the correct option.
• The rear wheel diameter decreased, and the front wheel diameter increased equally: This would increase the ratio, causing more front wheel revolutions for each rear wheel revolution. This is incorrect.

Therefore, the correct answer is: Both the rear and front wheel diameters are increased equally.