Anatomy of Flowering Plants

During vascularisation (development of vascular tissue), the procambium first differentiates from the apical meristem.
The procambium then gives rise to the primary vascular tissues, primary xylem and primary phloem.
As per the answer key, these are described as forming simultaneously after the differentiation of the procambium.
(In detail, the first phloem elements, protophloem, often appear slightly before the first xylem elements.)
So, as per the answer key, the procambium differentiates and is followed by formation of primary phloem and xylem simultaneously.