Which properties, physical or chemical, were most useful in identifying the mystery powder? explain your answer.

The answer depends on what substances you were testing, but here are some ideas.

If all of them were white as in the typical experiment, then color is not very useful.  If you were allowed to taste them, then a salty taste would be a helpful clue.  If the textures were different, some crystals, like salt or sugar and others powdery, like flour or cornstarch, those properties would be helpful.  If some dissolved in water,...

c. because step up transformer steps up the input voltage i.e increases the input voltage  

the number of turns are greater on the secondary side of step up transformer  

transformer does not handle dc current

the answer to this question is 3.