When the sepoys began their action with a signal, what is the first thing they did during the Revolt of 1857?

Seized the bell of arms

The correct answer is Option (4) → Seized the bell of arms

How the mutinies began:

The sepoys began their action with a signal:

1. In many places it was the firing of the evening gun or the sounding of the bugle.
2. They first seized the bell of arms and plundered the treasury.
3. They then attacked government buildings – the jail, treasury, telegraph office, record room, bungalows – burning all records.

Everything and everybody connected with the white man became a target. Proclamations in Hindi, Urdu and Persian were put up in the cities calling upon the population, both Hindus and Muslims, to unite, rise and exterminate the firangis.