Which of the following is the sixth planet from the sun, and what is a proof that the Earth is not flat?

Here are the solutions to the questions:

25. According to distance from the sun, which is the sixth planet?
The order of planets from the Sun is Mercury, Venus, Earth, Mars, Jupiter, and then Saturn. Saturn is the sixth planet.
The correct option is A.
The sixth planet from the sun is $\boxed{\text{A. Saturn}}$.

26. Which one of the options is a proof that the earth is not flat?
The phenomenon of a ship disappearing hull-first over the horizon, with its mast remaining visible for longer, is a classic observation that demonstrates the Earth's curvature. If the Earth were flat, the entire ship would simply get smaller until it vanished.
The correct option is A.
A proof that the earth is not flat is $\boxed{\text{A. Ship's visibility}}$.

27. At what time of the year do you have Spring equinox?
The Spring (or Vernal) Equinox in the Northern Hemisphere occurs around March 20th or 21st, when the Sun crosses the celestial equator going northward. At this time, day and night are approximately equal in length across the globe.
The correct option is C.
The Spring equinox occurs on $\boxed{\text{C. March 21st}}$.

28. Fold Mountains are formed by only one of the given options.
Fold mountains are created when two tectonic plates move towards each other (converge), causing the Earth's crust to buckle, fold, and uplift. This process is driven by immense compressional forces.
The correct option is B.
Fold Mountains are formed by $\boxed{\text{B. Compressional forces}}$.

29. Relative Humidity is measured in the field by the use of what instrument?
A hygrometer is an instrument specifically designed to measure the amount of humidity or water vapor in the atmosphere.
The correct option is D.
Relative Humidity is measured by a $\boxed{\text{D. Hygrometer}}$.

30. Which river is not a tributary of River Niger?
The Benue, Kaduna, and Sokoto rivers are all major tributaries that flow into the River Niger. The Imo River, however, is a separate river system in southeastern Nigeria that flows directly into the Atlantic Ocean, not into the Niger River.
The correct option is A.
The river that is not a tributary of River Niger is $\boxed{\text{A. Imo}}$.

31. The formation of coal is associated with
Coal is a fossil fuel formed over millions of years from the remains of ancient plants. These plant materials were buried under layers of sediment and subjected to intense heat and pressure deep within the Earth's crust, leading to their transformation into coal.
The correct option is D.
The formation of coal is associated with $\boxed{\text{D. Heat and pressure}}$.