Concept: The Indian Penal Code gives an extended meaning to the word “act”. It includes both acts and illegal omissions.
Step 1: Understand single act.
A single act is naturally included in the meaning of act.
\[
A = \text{Included}
\]
Step 2: Understand series of acts.
The word act also includes a series of acts.
\[
B = \text{Included}
\]
Step 3: Understand illegal omission.
IPC provides that words referring to acts done also extend to illegal omissions.
\[
C = \text{Included}
\]
Step 4: Reject legal omission.
A legal omission is not punishable. Only illegal omission, where there is a legal duty to act, is relevant.
\[
D = \text{Not included}
\]
Therefore, the correct answer is:
\[
A,\ B,\ C \text{ only}
\]