Taxes on railway freights and fares:
Taxes on railway freights and fares are a form of union tax. These taxes are levied by the central government on the transportation of goods and passengers by railways. The revenue generated from these taxes goes to the central government's coffers.

Stamp duties on financial documents:
Stamp duties on financial documents are also a type of union tax. These duties are imposed by the central government on various financial transactions such as share transfers, loan agreements, promissory notes, etc. The revenue collected from stamp duties contributes to the central government's revenue.

Tolls:
Tolls, on the other hand, are not a form of union tax. Tolls are charges levied by the government on the usage of certain infrastructure facilities such as roads, bridges, tunnels, etc. These charges are generally collected by the state or local government and are used for the maintenance and development of the infrastructure.

Explanation:
The correct answer is option 'D' - A and B only. This is because taxes on railway freights and fares (option A) and stamp duties on financial documents (option B) are both forms of union tax. These taxes are levied by the central government and the revenue generated from them goes to the central government's revenue pool.

Tolls (option C), on the other hand, are not a type of union tax. They are charges levied by the state or local government for the usage of specific infrastructure facilities. The revenue collected from tolls is typically used for the maintenance and development of the infrastructure.

In summary, taxes on railway freights and fares as well as stamp duties on financial documents are forms of union tax, while tolls are not.