Picture a diode as a one way gate for current, then trace what happens to AC passing through it.
AC voltage swings between positive and negative many times a second.
During the half cycle that forward biases the diode, current flows through easily.
During the other half cycle, the diode is reverse biased and blocks the current almost completely.
The result is current that only ever flows in one direction, even though its size still varies, and this one direction current is called direct current.
Since the rectifier's job is exactly this change from a reversing current to a one direction current, it converts AC to DC, so option B is correct.