When two or more capacitors are connected in series across a potential difference:

a) the potential difference across the combination is the algebraic sum of the potential differences across the individual capacitors.
b) the equivalent capacitance of the combination is less than the capacitance of any of the capacitors.
c) each capacitor carries the same amount of charge.
d) all of the above choices are correct.

A) and B) are correct.

Explanation:

Let's take a look at the attached picture. Now

The total voltage across both capacitors is the same as the sum of the voltage from each device, that statement is true for any electrical device connected in series. So a) is TRUE

The equivalent capacitance is going to be:

And that value can be mathematically proven that is always less than any of the values of each capacitor. So b is TRUE

And through both capacitors flow the same current, but the amount of charge depends on the value of the capacitors, so only could be the same if the capacitors are the same value. Otherwise, don't. C) not always, so FALSE