Which element is located at the centre of the porphyrin ring in chlorophyll : -

A chlorophyll molecule has a tetrapyrrole (porphyrin) head and a long phytol tail.
A magnesium atom is located at the centre of the porphyrin ring, bonded to four nitrogen atoms.
Magnesium deficiency therefore causes chlorosis (yellowing of leaves).
(In haem of haemoglobin, the central atom is iron.)
So the element at the centre of the porphyrin ring of chlorophyll is magnesium.