Concept:
Jelly making depends on the formation of a gel structure. The important components are pectin, sugar, acid and water.
Step 1: Identify the gel-forming substance.
Pectin is the substance responsible for gel formation in jelly.
\[
\text{Pectin} \rightarrow \text{gel formation}
\]
Step 2: Role of sugar and acid.
Sugar helps in preservation and texture. Acid helps pectin form a proper gel. But the main essential gelling constituent is pectin.
Step 3: Final answer.
Therefore, the most essential constituent for jelly making is pectin.
\[
\therefore \text{Correct Answer is (B)}
\]