The man's heart attack scared him badly and he decided having a regular check-up from then on.

The choice to select "having" as the correct answer relates to the grammatical structure of the sentence. The phrase "decided having a regular check-up" is not standard in English. The use of "having" suggests a gerund form which is not appropriate following the verb "decided." Instead, the correct form should be "decided to have a regular check-up." This change clarifies the intention of the sentence, indicating that the man made a decision to perform the action of having a regular check-up, rather than implying that in the moment of making a decision he was in a state of having.

The other options, while they may contain words that can exist correctly in different contexts, do not relate to the grammatical flaw in the sentence structure in the way that "having" does. Therefore, selecting "having" highlights the need for clear and grammatically correct constructions following certain verbs.