What has happened to Thailand's constitutional government over the last two decades?

The correct choice reflects the significant political volatility experienced by Thailand over the last two decades. The constitutional government in Thailand has seen several coups, political protests, and shifts in government that have led to a tumultuous political landscape. Since the early 2000s, there have been notable events, such as the military coup in 2006 that ousted then-Prime Minister Thaksin Shinawatra, and another coup in 2014, which further destabilized the already fragile political system. These events contributed to widespread perceptions of instability, characterized by intermittent periods of military rule and contested elections.

In contrast, the other options suggest various degrees of stability, which do not accurately reflect the reality of Thailand's political situation. The assertion of increased or maintained stability does not take into account the repeated disruptions that have fundamentally challenged the constitutional processes in the country. Hence, the option indicating that the government has become unstable accurately captures the ongoing conflicts and challenges faced within Thailand's political framework.