In a grounded grid amplifier using a triode vacuum tube, the secondary winding of a transformer is connected directly to the vacuum tube. This transformer provides:

In many high-power transmitting triodes, the tube uses a directly heated cathode. This means the filament itself acts as the electron emitter when it gets hot. To heat the filament, a step-down transformer is used to provide a low-voltage, high-current AC supply directly to the tube's filament pins. This raw AC voltage from the transformer secondary is the filament voltage. The other options are incorrect for a direct transformer connection. B+ and B- are DC voltages that require rectification and filtering before reaching the tube. Furthermore, a triode by definition only has three active elements (cathode, control grid, plate) and does not have a screen grid.