How is the mechanical efficiency of a machine usually expressed 1. as a fraction 2. as a percentage 3. as inches or feet 4. as one of three classes

The efficiency of any contraption that uses energy to
do a job is defined as

         (amount of useful energy output) / (amount of energy input) .

In describing or talking about the efficiency f the machine,
it's most often expressed as a percentage.

It's important to notice that the output is taken to be the useful output.
Why is that important ? 
Here's a simple example:

If you use an incandescent light bulb as a source of light, then
the useful output is the light, and its efficiency is quite low.

If you use an incandescent light bulb as a source of heat, then
the useful output is the heat, and its efficiency is quite high.

The best answer is (a) which is universal law is in effect at all times. it is constantly at play in everything we do and governs our lives. it is also important to note that universal law applies to not only everything on earth, but everything in the universe as well, which is another reason why it is in effect at all times.