The most accurate response to the question "Why did industrialization spread to other countries?" is:
Britain lost their industrial secrets due to espionage.
While other factors contributed to the spread of industrialization, the idea that industrial secrets were acquired through espionage or the movement of people (such as British artisans and engineers) to other countries played a significant role in this process.
However, it's worth noting that the other responses also contain elements of truth. Industrialization was indeed influenced by the desire of other countries to replicate Britain's success, and in some cases, individuals from Britain did carry ideas abroad. Overall, espionage highlights the dynamics of information transfer that facilitated industrial growth elsewhere.