(a) Sperm A would fertilise the ovum as it initiated the acrosomal reactions of fertilisation by dissolving the ground substances and corona radiata of ovum. Sperm A will further dissolve the zona pellucida using sperm lysins and plasma membrane of sperm fuse with the plasma membrane of ovum so that the sperm contents enter the oocyte.
Binding of sperm to the ovum induces depolarisation of plasma membrane of oocyte which would prevent polyspermy, i.e., entry of more than one sperm into oocyte, thus, sperms B and C would not be able to fertilise the ovum and would degenerate.
(b) Zona pellucida is a thick, acellular, outer layer of egg. It is vital for acrosomal reaction, which helps sperm to penetrate the egg during fertilisation. It induces changes in membrane when interact with sperm to prevent polyspermy.
(c) Ovum (secondary oocyte) completed the second meiotic division after the entry of sperm and forms one haploid ovum (ootid) and 2nd polar body.
Entry of capacitated sperm into ovum is facilitated by various chemicals called sperm lysin, such as
(i) hyaluronidase - acts on ground substance of follicular cells,
(ii) corona penetrating enzyme-dissolves corona radiata and
(iii) zona lysins - digests zona pellucida.
(d) Fertilisation takes place in ampullary - isthmic junction of Fallopian tube
