what is the best route from sosua to boca chica by rental car.how long would it take?
It depends very much in what time of the day you cross Santo Domingo.
Rush hour can add one extra hour.
Normal driving without traffic jams would have you there in 4 hours.
Going to Sabaneta, South to Moca, over to the Duarte highway, then onto Santo Domingo would be the preferred route, but if you are not familiar with getting through Moca, it may be better for you to go by the Caretera Turistica at Montellano, and through Santiago.