1. Identify the reaction of nitrile with Grignard reagent

A nitrile reacts with one equivalent of Grignard reagent to give an imine magnesium salt, which on hydrolysis gives a ketone.

For benzonitrile: $C_6H_5CN + CH_3MgBr \rightarrow C_6H_5C(=N\!MgBr)CH_3$ On hydrolysis: $C_6H_5C(=N\!MgBr)CH_3 \xrightarrow{H_3O^+} C_6H_5COCH_3$

So the product $P$ is: $P = C_6H_5COCH_3$ This is acetophenone (methyl phenyl ketone), a yellow liquid.

2. Test the product against the given options

(A) Iodoform test

Iodoform test is given by compounds containing the group: $-COCH_3$ Acetophenone has the structure: $C_6H_5-CO-CH_3$ So it contains a methyl ketone group and gives positive iodoform test.

(B) Schiff's test

Schiff's reagent gives positive test mainly with aldehydes. Acetophenone is a ketone, so this is negative.

(C) Ninhydrin's test

Ninhydrin test is given by amino acids / primary amines in special cases, not by acetophenone. So negative.

(D) Tollen's test

Tollen's reagent is reduced by aldehydes (and a few special compounds like formic acid, alpha-hydroxy ketones). Acetophenone does not give this test. So negative.

3. Conclusion

The product $P$ is acetophenone, which gives a positive iodoform test.

Therefore, the correct option is: $\boxed{A\text{: Iodoform test}}$

4. Comparison with stored correct answer

Stored correct answer: A

My derived answer: A

They agree.