In general the segment from A to B has parametric equation
X = (1-t)A + tB
t=0 gives point A, t=1 gives point B, and in between we move linearly with t from A to B.
So if we want AX:BX=1:3, that's 1/(1+3)=1/4 of the way along from A to B, so corresponds to t=1/4. So the point we seek is
X = (1 - 1/4)(3, 2) + (1/4)(6,8) = ((9+6)/4, (6+8)/4)=(15/4, 7/2)
Answer: Choice C