i do not think there's any direct formula involved here...i know a method to solve this problem..the answer is (a) 1.4...
I do not know if there is an alternate method to solve this but this is the one i thought of.
you should probably be drawing a diagram for this one.....basically, the given equation of the circle is that of a circle with a radius of 1 and center at the origin(0,0)(call this point O)(draw this, seriously!!).
The given line's equation give you two points on the X and the Y axis. The two points will be (4,0) on the X axis(call this one point A) and (0,-3) on the Y axis(call this one point B).
Now the least possible distance would be a perpendicular from the circle to the line...in other words, a perpendicular from the center of the circle(the origin) to the given line......this perpendicular line would have a slope of (-4/3) and an equation of y=(-4X/3).....using the equations of the given line and the perpendicular, we get an intersection between the two lines at point C(1.44,1.92).
If you have drawn a diagram for this, you will see that there is a right angled triangle formed between the points of the given line intersecting the X and the Y axis and the origin. ie. triangle OAB. According to the rules of geometry (OC)^2=(AC)*(BC).
i.e..(OC)^2=5.76
therefore OC=2.4 units.
Do not forget to subtract the radius from this length.
Therefore,least possible distance is 2.4-1=1.4.
Hope that helps.Good luck.