The correct choice is (d) 1 + βdc
The explanation is: Stability factor is fundamentally dependent on the ratio of base resistance to emitter resistance(RB / RE). Hence, if this ratio is estimated to be small, then the stability factor in such case is equal to unity.
On the contrary, if this ratio RB / RE tends to infinite value, then the value of stability factor in voltage divider circuit becomes nearly equivalent to 1+ βdc.
Therefore, if the ratio is constant then it leads to an increase the stability factor by increasing the value of current gain (βdc). Besides this, smaller values of base resistance also yield improved stabilization.