What was the planet last to be discovered out of these?

1.Mars
2.Mercury
3.Neptune
4.Pluto
5.Saturn
6.Uranus
7.Venus

Only 3 of the 9 planets in our solar system have official "discoverers" and "times of discovery".

Uranus was discovered by Sir William Herschel in 1781.
Neptune was discovered by John Couch Adams in 1846.
Pluto was discovered by astronomer Clyde Tombaugh in 1930.

The last planet to be discovered out of the options provided is Neptune. To determine this without prior knowledge, you can follow these steps:

1. Start with the earliest known planet: You know that the Earth and Venus are not the answer, as they are widely known and appear in ancient astronomy records.

2. Consider the solar system's inner planets: Mercury, Venus, Earth, and Mars. These were known to ancient astronomers due to their visibility and proximity to the Sun, making them less likely options.

3. Move to the outer planets: Saturn and Jupiter. These planets were also known to ancient civilizations due to their size and brightness.

4. Focus on the remaining outer planets: Uranus and Neptune. Uranus was discovered first, but the question asks for the planet that was discovered last.

Therefore, out of the given options, the planet last to be discovered is Neptune.