Solution

Roots lack secondary or lateral cambium whose activity results in secondary growth or increase in girth. So, in all roots including dicot roots the secondary growth is of the anomalous type. The cambium is formed by dedifferentiation of living components of the phloem tissue (phloem parenchyma) and the cells of the pericycle, that produce secondary phloem and secondary xylem.
So, the correct answer is 'pericycle'