Which one of the following metal salts produces a blue coloured bead in cobalt nitrate charcoal cavity test?

Aluminium metal salts produce a blue coloured bead in the cobalt nitrate charcoal cavity test.

$A{l}_2{O}_3.COO$ formed in the test is blue. It is called thenard's blue.

This precipitate is coloured blue by the absorbing of litmus solution. In this test, the reaction between the aluminium oxide generated in the charcoal cavity test and the cobalt nitrate test results in the creation of $A{l}_2{O}_3$ and a blue mass. 

Hence, option D is correct.