In K-Map, those groups which cover at least one minterm that can't be covered…

In K-Map minimization, a prime implicant is a group of adjacent minterms that cannot be combined further. An essential prime implicant is a specific type of prime implicant that covers at least one minterm not covered by any other prime implicant. This minterm is unique to that group, making the prime implicant essential for the minimal sum-of-products expression.
To identify essential prime implicants:
Look for minterms that are covered by only one prime implicant.
The prime implicant covering such a minterm is essential.
These must be included in the final minimized expression.
Non-essential (redundant) prime implicants may be omitted if all minterms are covered by other implicants.