Which of the following statements is/are not correct ?

Options

A. A substance which can provide `OH^(-)` in aqueous medium is a base
B. A substance which can accept a pair of electronis a base
C. A sumstance which can accept a proton in aqueous medium is a base
D. A substance which can donate a pair of electron is a base

Answer

To determine which statements are not correct regarding bases, we will analyze each statement one by one.

### Step 1: Analyze the First Statement
**Statement:** A substance which can provide OH⁻ in aqueous medium is considered as a base.

**Analysis:** This statement is true. According to the Arrhenius definition of bases, a substance that increases the concentration of hydroxide ions (OH⁻) in an aqueous solution is classified as a base.

### Step 2: Analyze the Second Statement
**Statement:** A substance which can accept a lone pair is known as a base.

**Analysis:** This statement is false. A substance that accepts a lone pair is classified as a Lewis acid, not a base. Therefore, this statement is incorrect.

### Step 3: Analyze the Third Statement
**Statement:** A substance which can accept a proton in an aqueous medium is a base.

**Analysis:** This statement is also false. A substance that accepts a proton (H⁺) is considered an acid, according to the Brønsted-Lowry definition. Therefore, this statement is incorrect.

### Step 4: Analyze the Fourth Statement
**Statement:** A substance which can donate a lone pair is a base.

**Analysis:** This statement is true. According to the Lewis definition of bases, a base is a substance that can donate a pair of electrons (lone pair) to form a bond. Hence, this statement is correct.

### Conclusion
The statements that are not correct are:
- A substance which can accept a lone pair is known as a base (False - it is a Lewis acid).
- A substance which can accept a proton in an aqueous medium is a base (False - it is an acid).

### Final Answer
The incorrect statements are the second and third statements.

---