No machine can achieve 100% efficiency in terms of work output. in any mechanical system, what is considered the most common cause for some of the work being turned into thermal energy? a) friction b) inefficiency c) poor design of the machine d) not using the machine properly

The answer is A, friction.

Friction is the force that resists movement, and that's why oil is used as a lubricant and why ball bearings are used for machines that need to move with very little friction(i.e. bike wheels, roller skates, fidget spinners)

Answer B is too vague
C will probably reduce efficiency greatly, but not specifically in a way that turns work into thermal energy.
D is the same as C, except that it places the blame on the person using the machine, not the machine itself.

Hope I helped clear it up for you :)

The answer is A, Friction. The other ones don't make sense

Friction- action of one surface or object rubbing against one another

Hope this helps!