**Increasing order of acid strength:**

1. Propan-1-ol
2. Phenol
3. 4-methylphenol
4. 3-nitrophenol
5. 3,5-dinitrophenol
6. 2,4,6-trinitrophenol

**Explanation:**

To determine the acid strength of the given compounds, we need to consider the factors that influence acidity. The main factors are the stability of the conjugate base and the strength of the bond between the acidic hydrogen and the molecule.

1. **Propan-1-ol:** Propan-1-ol is an alcohol and does not have any acidic hydrogen. Therefore, it is not acidic and has the lowest acid strength among the given compounds.

2. **Phenol:** Phenol has a hydroxyl group attached to a benzene ring. The presence of the benzene ring increases the stability of the phenoxide ion formed after deprotonation. The delocalization of the negative charge across the ring contributes to the stability of the phenoxide ion. Therefore, phenol is more acidic than propan-1-ol.

3. **4-methylphenol:** 4-methylphenol, also known as p-cresol, has a methyl group attached to the para position of the benzene ring. This methyl group destabilizes the phenoxide ion by increasing the steric hindrance and reducing the delocalization of the negative charge. As a result, 4-methylphenol is less acidic than phenol.

4. **3-nitrophenol:** 3-nitrophenol has a nitro group (-NO2) attached to the meta position of the benzene ring. The electron-withdrawing nature of the nitro group destabilizes the phenoxide ion by withdrawing electron density from the ring. This reduces the delocalization of the negative charge, making 3-nitrophenol more acidic than 4-methylphenol.

5. **3,5-dinitrophenol:** 3,5-dinitrophenol has two nitro groups attached to the meta positions of the benzene ring. The presence of two nitro groups further increases the electron-withdrawing effect, making 3,5-dinitrophenol more acidic than 3-nitrophenol.

6. **2,4,6-trinitrophenol:** 2,4,6-trinitrophenol, also known as picric acid, has three nitro groups attached to the ortho positions of the benzene ring. The electron-withdrawing effect of the nitro groups is at its maximum in this compound, making it the strongest acid among the given compounds.

In summary, the acid strength increases in the following order: Propan-1-ol < phenol="" />< 4-methylphenol="" />< 3-nitrophenol="" />< 3,5-dinitrophenol="" />< 2,4,6-trinitrophenol.="" />