[tex]\dfrac{3}{4}a+\dfrac{1}{2}b-\dfrac{1}{2}a=\left(\dfrac{3}{4}a-\dfrac{1}{2}a\right)+\dfrac{1}{2}b\\\\\text{Find the common denominator:}\ it's\ 4\\\\\dfrac{1}{2}=\dfrac{1\cdot2}{2\cdot2}=\dfrac{2}{4}\\\\=\left(\dfrac{3}{4}a-\dfrac{2}{4}a\right)+\dfrac{1}{2}b=\boxed{\dfrac{1}{4}a+\dfrac{1}{2}b}\to\boxed{A)}[/tex]
