Solution

" Significant " is the most appropriate word to fill in the blank because it conveys the importance and considerable impact of early childhood experiences on brain development, aligning with the idea of affecting multiple aspects of life. (2) Trivial: This means insignificant or minor, which does not match the context of a major impact. (3) Negligible: This indicates something of little importance or impact, which does not fit the context of significant influence. (4) Transient: This means temporary or short-lived, which is contrary to the idea of a long-lasting influence. Thus, "significant" is the best choice for describing the substantial and lasting impact of early childhood experiences.