Which planet has nearly identical length of day as Earth?

Mars has an equal length of day as Earth.
The daily cycle on Mars is remarkably similar to that on Earth. The solar day lasts 24 hours, 39 min, 35 sec, and the sidereal day is 24 hours, 37 min, 22 sec. 

Thus, a Martian day (known as "sol") is about 40 minutes longer than an Earthly day. Mars has a tilt of 25^o, which is very close to the Earth's tilt of 23.5^o. The orbit of Mars is also extremely elliptical. This causes a 19% variation in the distance between its closest and farthest points along its orbit.

Hence, "Mars" is the correct option.