Light Years.
To measure the distance of stars you have to measure by light years.
The correct unit to describe the distance between stars is the "light year", the distance a photon of light would travel (through a vacuum) in a year.
the measurer
The correct answer would be 40.44.
You would need the individual orbital period of the stars around each other and the speed at which they orbit. These parameters would allow you to apply Kepler's laws of planetary motion to calculate the masses of the stars in the system.
you would use kilometers.
Light years, are basically how far would does light travel, at roughly 186,000 miles a second, if it were to go for a year.
Scientists measure light from star in light years because almost every other units would be so small compared to the distance involved between stars. in fact the nearest star (other then the sun) is 24689699219682.054 miles or 39734219301000 km.
A ruler
To measure the wavelength of a transverse wave, you would typically measure the distance between two consecutive points that are in phase with each other, such as two crests or two troughs. This distance would represent one full wavelength. Alternatively, you could measure the distance between the start and end points of one complete wave cycle.
Km is used for long distances. As the distance between two cities.
Any of those units could be used. 'Kilometers' would result in the most convenient number.