Which of the following is true for balanced chemical equations?

d

D!

C is always true.
A could be gaseous, for example. H2 + 02 ===> H20 (This is not balanced yet. I'm just giving an example)
D I'm not sure what this means, but generally you multiply subscripts when they are outside the brackets (for example).
B. Need not be true.

D. Subscripts are added to atoms or formulas to balance equations.

the answer is most definitely "C"

2 C8H18 + 25 O2 ---> 16 CO2 + 18 H2O is an example of a balanced chemical equation that occurs in your cars engine.  This s NOT an aqueous based reaction.

equilibrium means the forward and reverse rates of the reaction are equal.  The reaction I gave you above is NOT an equilibrium reaction.  The rate of CO2 + H2O ---> C8H18 + O2 is NOT= rate of C8H18 + O2 ---> CO2 + H2O.

Subscripts are NEVER added to formulas to balance equations.  You don't balance the above equation like this do you?
C16H36 + O50 --->  C16O32 + H36O18