Correct choice is (a) obturator
To elaborate: In the ovary the growth of the pollen tube is directed by another tissue called obturator. The pollen tube enters the ovule, either through its micropyle (porogamy e.g., Lily), chalaza (chalazogamy e.g., Betula, Casuarina, Juglans regia) or the sides after piercing through the integuments (mesogamy, e.g. Cucurbita, Populus, Pistacea).