Draw a green diameter perpendicular to the red diameter, which biscects 
chord AB at E into two 3 cm parts, chord CD at F into two 4 cm parts, and the
red diameter into two 5 cm radii 



Draw in radii OA and OC, which are 5 cm each.


 
We use the Pythagorean theorem on right triangles OAE and OCF to find OE and
OF

   OA² = AE² + OE²           OC² = CF² + OF²
    5² =  3² + OE²            5² =  4² + OF²
    25 =   9 + OE²            25 =  16 + OF²
25 - 9 = OE²             25 - 16 = OF²
    16 = OE²                   9 = OF²  
     4 = OE                    3 = OF



The distance between the chords = EF = OE + OF = 4 + 3 = 7cm.

Therefore it is none of the choices you gave.  The correct distance
is 7 cm.

Edwin