Your answer is

but if you are looking for some explanation check description below :)
Diphosphorus trioxide formula is:

Di stands from 2 atoms of something, in this case we have 2 atoms of Phosphorus.
Trioxide means that we have 3 atoms of oxygen in this substance that's why we have:

And water is just

Unbalanced formula is:

Now we need to balance this equation
On left hand side we have 2 atoms of Phosphorus, on right hand side we have just one atom of Phosphorus, that's why we need to put 2 before

Now we need to count all atoms of oxygen and hydrogen and make it equal on both side.
So we need to put 3 on left hand side before water to make this equation balanced.
That's why correct answer is:
Answer:
c. 0.750 atm
.
Explanation:
Hello!
In this case, since the two vessels have different volume, we can see that the gas is initially at 3.00 atm into the 1.00-L vessel, but next, it is allowed to move towards the 3.00-L vessel, meaning that the final volume wherein the gas is located, is 4.00 L; therefore, we use the Boyle's law to compute the final pressure:

Therefore the answer is c. 0.750 atm
.
Best regards!
Answer:
c. add coefficients as needed
Explanation:
A chemical equation is defined as the equation that shows changes in a chemical reaction. A chemical equation consist of reactant and product, reactant is at left side of the arrow and product is at right side of the arrow.
Reactant => Product
While balancing a chemical equation, the basic rule is to balance the coefficient as required. Coefficient represents the number of molecules and is used at front of a chemical symbol. Change in coefficient helps balance the number of atoms or molecules of the substances on both the sides of the arrow.
Subscripts are never allowed to change because it can change the chemical involved in the reaction.
Hence, the correct answer is "c. add coefficients as needed".