Concept:Integration by parts is applied repeatedly to reduce the power of x until only a simple integral remains.Explanation:Use the formula ∫udv=uv−∫vdu.Take u=x3 and dv=cosxdx.Then du=3x2dx and v=sinx.So, ∫x3cosxdx=x3sinx−3∫x2sinxdx.Apply parts again on ∫x2sinxdx with u=x2 and dv=sinxdx.This gives −x2cosx+2∫xcosxdx.Now ∫xcosxdx=xsinx+cosx+c.Substituting back, the full integral simplifies to x3sinx+3x2cosx−6xsinx−6cosx+c.This matches option C.Answer:Option C