The IC 555 timer contains an internal resistive voltage divider made of three equal resistors (each typically 5 kΩ) connected across the supply. This divider produces two fixed reference voltages: 1/3 V_cc and 2/3 V_cc. These references feed two internal comparators whose outputs drive an SR (Set-Reset) flip-flop, which in turn controls the output (Pin 3) and the discharge transistor (Pin 7).

The component that resets the flip-flop when Pin 6 (Threshold) exceeds 2/3 V_cc is the Upper Comparator:

• Its input compares the Threshold voltage at Pin 6 against the 2/3 V_cc reference from the divider.
• When the Pin 6 voltage rises above 2/3 V_cc, the Upper Comparator output goes HIGH.
• This HIGH signal RESETS the internal flip-flop, which turns the discharge transistor ON and drives the output LOW.

Why the other choices are wrong: the Lower Comparator does the opposite job — it monitors Pin 2 (Trigger) against 1/3 V_cc and SETS the flip-flop, so it cannot be responsible for resetting. The voltage divider network only generates the reference levels; it is a passive resistor chain and performs no comparison or switching. The discharge transistor is the output-side device that is switched ON after the flip-flop is reset — it is an effect of the reset action, not the cause of it.