One of the lines of Hydrogen has a wavelength of 486.1 nm. If a cloud of Hydrogen gas is moving towards us at 100 km/sec, what will we detect?

An emission line in the cloud's spectrum with a wavelength shorter than 486.1 nm.

Explanation:

According to the Doppler's effect, when an object is moving toward observer will cause to reduce its wavelength and increase in the frequency.

In this case, when a cloud of hydrogen gas is moving toward us, each followed emitting wave will be closer than the previous one. While traveling at a constant velocity, the time between the hydrogen cloud and us will be reduced. In reduced time, new emitted lime in the cloud spectrum has a shorter wavelength than 486.1 nm.