The correct answer is option 1: Tropical Wet Evergreen - Tropical Moist Deciduous - Tropical Dry Deciduous - Tropical Thorn Forest.

When moving from Assam Valley to Rajasthan Plains, the sequential order of vegetation types can be observed as follows:

1. Tropical Wet Evergreen: This vegetation type is found in areas with high rainfall and high humidity. It is characterized by dense and lush evergreen forests.

2. Tropical Moist Deciduous: As we move away from Assam Valley to regions with slightly lower rainfall, we encounter tropical moist deciduous forests. These forests have trees that shed their leaves during the dry season.

3. Tropical Dry Deciduous: Moving further towards Rajasthan Plains, where the rainfall decreases significantly, we encounter tropical dry deciduous forests. These forests have trees that shed their leaves during the dry season, but they are more adapted to drought conditions compared to tropical moist deciduous forests.

4. Tropical Thorn Forest: Finally, in Rajasthan Plains where the rainfall is minimal, we come across tropical thorn forests. These forests are characterized by thorny shrubs and drought-resistant vegetation.

Therefore, the correct order of vegetation types, observed while moving from Assam Valley to Rajasthan Plains, is Tropical Wet Evergreen - Tropical Moist Deciduous - Tropical