Please refer to which question u r answering.
a.The brightness of the bulb is a measure of the energy that the bulb is using. As u add batteries to the circuit, what happened to the total amount of energy used by the bulb?

b.sometimes batteries are hooked in series in electrical devices such as a flashlight. What does the investigation indicate as a possible reason for doing this?

c.the anmeter measures the potential difference across the light bulb.This is the difference in energy that each coulomb of charge has on one side of the light bulb as compared to the other side.Wut happened to the potential difference across the light bulb as more batteries were added,in series, to the circuit? Why would u have expected this?

d.the anmeter measures the amount of charge flowing through the bulb.wut happens to the current as you add more batteries?In other words, what happens to the light bulb as you increase the potential difference across the bulb?

a. It all depends on how you add the batteries.

b. Applicable to above. Adding batteries in serial increases the voltage you're capable of using. If you have 3 AA 1.5 volts batteries in series...you have 4.5 volts to work with. This ups the current too if the resistance load stays the same. To get more amp hours without continuously raising the current you can add another stack similar stack of 3 AA batteries in parallel. If you connect those packs in parallel you now have more amp hours to work with. So if you have a bulb hooked up to the paralleled battery packs, it'll stay lit longer than with just the single battery pack but it won't overload the bulb like adding more batteries in serial will eventually do.

c. You can figure this out with Ohm's Law: V=IR

V = voltage
I = current
R = resistance

Think of a light bult as a resistor. You can calculate it resistance by using VI=P or (voltage * current (in amps) = power (Watts).

If you keep adding more batteries in series the voltage adds up as does the current. So us the above laws to find your answer.

d. I'm guessing the bulb will get brighter and brighter until you burn out the bulb; that is if you add more batteries in serial.