Final answer:
An essential prime implicant is a prime implicant that covers at least one minterm not covered by any other prime implicants, making it necessary for the function's minimal sum-of-products expression.
Step-by-step explanation:
To determine which prime implicant is essential, first identify all prime implicants of a function in a Karnaugh map or a truth table. A prime implicant is a group of ones which can be combined according to the laws of Boolean algebra, and cannot be combined with any other groups to cover more ones. An essential prime implicant is a prime implicant that includes at least one minterm (a specific combination of variable values producing a value of one) that is not covered by any other prime implicant. In other words, this minterm is unique to that implicant alone. Therefore, that particular prime implicant becomes necessary for the function's minimal sum-of-products expression, ensuring that every minterm of the function is covered.