Mariner 10 was the first to visit this planet in 1974. Which planet is this?

Answer: Mercury

The Mariner 10 probe was launched by NASA on November 3rd, 1973, with the purpose of exploring the characteristics of two planets in the solar system that were closest to the Sun, Mercury and Venus.

In addition, it was launched to explore the atmosphere and surface of both planets and prove that it was possible to use gravitational assistance (also called slingshot effect, a special orbital maneuver in order to use the gravitational field energy of a planet or massive body to accelerate or slow the probe and change the direction of its trajectory) in long interplanetary trips to save fuel.

In this case, Mariner 10 first arrived at Venus and succeded in using its gravitational field to accelerate its trajectory towards Mercury.