The events after fertilization occur in the following sequence:

Step 1: Formation of Morula (D)
After fertilization, the zygote undergoes repeated mitotic divisions (cleavage).
It forms a solid ball of 8–16 blastomeres, known as the morula.

Step 2: Formation of Blastocyst (C)
The morula develops into a blastocyst.
In the blastocyst:
The outer layer differentiates into the trophoblast.
The inner group of cells forms the inner cell mass (ICM), which develops into the embryo.

Step 3: Implantation (A)
Around 6–7 days after fertilization, the blastocyst attaches to and becomes embedded in the endometrium of the uterus.
This process is called implantation.

Step 4: Formation of Chorionic Villi (B)
After implantation, the trophoblast develops finger-like projections called chorionic villi.
These villi help form the placenta, enabling the exchange of nutrients, oxygen, and waste products between the mother and the developing fetus.

Intrauterine Devices (IUDs) are contraceptive devices inserted into the uterus to prevent pregnancy. According to NCERT, IUDs are of three main types:

Non-medicated IUDs: Lippes Loop
Copper-releasing IUDs: Cu-T, Cu-7, Multiload 375
Hormone-releasing IUDs: LNG-20, Progestasert

Now evaluate each option:

(A) Lippes Loop
A non-medicated intrauterine device (IUD).
Involved in IUDs.
(B) LNG-20
A hormone-releasing IUD containing levonorgestrel.
Involved in IUDs.
(C) Saheli
Saheli (Centchroman/Ormeloxifene) is an oral contraceptive pill, not an IUD.
Not involved in IUDs.
(D) Implants
Hormonal contraceptive rods placed under the skin (subdermal implants).
They are not intrauterine devices.

This question is based on the Lac Operon model proposed by Jacob and Monod, explaining gene regulation in E. coli.

Correct Matching

Gene: Encodes (Function)
i: Repressor protein (Binds the operator and blocks transcription in the absence of lactose)
z: β-Galactosidase (Breaks lactose into glucose and galactose)
y: Permease (Facilitates the entry of lactose into the bacterial cell)
a: Transacetylase (Transfers acetyl groups; its exact physiological role is less significant in lactose metabolism)