Which term means to express a positive change that is hard to put into words?

The term that refers to expressing a positive change that is hard to put into words is "tacit." Tacit denotes something that is understood or implied without being explicitly stated. This makes it particularly suitable for describing positive changes that might evoke feelings or realizations that individuals might struggle to articulate clearly. In contexts where emotional or nuanced transformations are experienced, the word captures the essence of understanding without the necessity of verbalizing those experiences.

In contrast, synthesis refers to the combination of ideas or elements to form a coherent whole, which does not inherently relate to expressing an indescribable change. Stymie means to hinder or obstruct progress, which is unrelated to the concept of expressing positive transformations. Lastly, stupefy means to stun or confuse, which also does not align with the idea of articulating an implicit positive change. Thus, the correct option accurately conveys the notion of a change that is beneficial yet difficult to express in precise terms.