The iodoform test:
The iodoform test is a chemical test used to detect the presence of a methyl ketone or a compound that can be oxidized to a methyl ketone. The test involves the reaction of the compound with iodine and sodium hydroxide in the presence of water. If the compound contains a methyl ketone group, it will produce a yellow precipitate of iodoform.

Explanation:
In this question, we are given four compounds - 1-pentanol, 2-pentanone, 3-pentanone, and pentanal. We need to identify which compound will give a positive iodoform test.

To determine the compound that gives a positive iodoform test, we need to consider the structure and functional groups present in each compound.

1. 1-pentanol:
1-pentanol is an alcohol and does not contain a methyl ketone group. Therefore, it will not give a positive iodoform test.

2. 2-pentanone:
2-pentanone is a methyl ketone (a ketone with a methyl group attached to the carbonyl carbon). It contains the required functional group to give a positive iodoform test. The reaction of 2-pentanone with iodine and sodium hydroxide will result in the formation of iodoform, which will precipitate as a yellow solid.

3. 3-pentanone:
3-pentanone is also a methyl ketone and contains the necessary functional group to give a positive iodoform test. Similar to 2-pentanone, the reaction of 3-pentanone with iodine and sodium hydroxide will produce iodoform.

4. Pentanal:
Pentanal is an aldehyde and does not contain a methyl ketone group. Therefore, it will not give a positive iodoform test.

Conclusion:
Among the given compounds, 2-pentanone and 3-pentanone will give a positive iodoform test. Therefore, the correct answer is option 'B' - 2-pentanone.

2-pentatone becoz only an acyl group can give positive iodoform test