An antifertility drug is:(a) novestrol(b) histamine(c) vernal(d) equanil

The correct answer is (a) **novestrol**.

### Explanation:

- **Antifertility drugs** are used to prevent pregnancy by inhibiting the process of fertilization or ovulation. These drugs are often hormonal in nature and work by regulating the reproductive cycle.

- **Novestrol** is a synthetic estrogen and is commonly used in oral contraceptives. It helps in preventing ovulation and is considered an antifertility drug.

- Let's briefly explain the other options:
- **Histamine** is a compound involved in immune responses, such as allergic reactions. It is not related to antifertility.
- **Vernal** is not a drug related to fertility or contraception. It refers more to seasonal changes.
- **Equanil** is a tranquilizer used to treat anxiety and is not associated with fertility control.

Hence, the correct option is **novestrol (a)**.