Answer: the fourth stament, The theoretical yield is calculated from the amount of the limiting reactant present.
Justification:
1) Limiting reactant is the one that is in less proportion than the theoretical mole ratio.
2) Excess reactant is the one that is in greater proportion than the theoretical ratio.
3) For example, if you have the equation A + 2B → C, means that 1 mol of A react with 2 moles of B to yield 1 mol of C.
As per that equation, the theoretical mole ratio is 1 mol of A : 2 moles of B : 1 moles of C.
If the actual amounts of A and B were 2 and 4, respectively, they were in the same ratio as the theoretical ratio, and all of the both reactants could react.
But if you had, for example, 2 moles of reactant A and 2 moles of reactant B, only one of the moles of A could react with the 2 moles of B, so B would be the limiting reactant: the whole 2 moles of B would react, while only 1 mole of A would reactant, and 1 mole of A would be left over (A is in excess).
Therefore, you cannot use the 2 moles of A to calculate the theoretical yield (since only 1 mole reacted), instead you must use the limiting reactant (B), which is the one that reacts completely.
