A circular coil has one turn and carries a current i. The same wire is bound into a smaller coil of 4 turns and the same current is passed through it. The field at the centre

decreases to ¼ of the value                                            is the same  increases to 16 times the value    decreases to 16 times the value

increases to 16 times the value

$B \propto (n/r)$             Here number of turns becomes 4 times while the radius becomes (1/4) times. Hence B becomes 16 times.