A basic moving-coil meter movement can safely carry only a very small current — here the full-scale current is just 1 mA with a coil resistance of 100 Ω. To measure larger currents (100 mA in this case), a low-value shunt resistor is connected in parallel with the movement. The shunt diverts most of the line current, so only the tiny rated fraction flows through the delicate coil.

In normal operation for a 100 mA range, when 100 mA flows in the line, about 1 mA passes through the movement and the remaining ≈99 mA passes through the shunt. The shunt is essential to protect the coil.

Now suppose the shunt goes open-circuit while a steady line current of 80 mA is flowing:

• The parallel bypass path is gone, so all 80 mA is forced through the meter movement.
• The movement is rated for only 1 mA, so it is subjected to about 80 times its rated current.
• This causes violent over-deflection and heavy I²R heating in the fine coil wire, which is very likely to burn out the coil or bend the pointer, damaging the movement.

Therefore the most likely outcome is that excessive current flows through the movement and damages it.

The other options are wrong: the reading does not drop to zero — losing the shunt sends more current through the coil, not less; the meter cannot continue to indicate correctly because the calibrated current division has been lost; and it does not simply become more sensitive with larger valid deflections — the pointer is driven past full scale into the destructive region, which is damage, not improved sensitivity.