To compare the ease/rate of diazotisation, we use the fact that diazotisation occurs for primary aromatic amines and depends mainly on how easily the amine can form the required nitrosated intermediate in acidic medium.

1. Key idea

In diazotisation, the amine reacts with nitrous acid in acidic medium:

The reaction is favored when the amine has suitable basicity and availability of the lone pair.

• If the lone pair is strongly withdrawn by resonance, diazotisation is slower.
• If the ring has substituents that make the amino group more available for reaction, diazotisation is easier.
• Strongly electron-withdrawing effects near the amino group can reduce reactivity.

2. Relative order for the given compounds

From the given choices, the only chemically consistent increasing order is:

This means:

• (a) diazotises least easily,
• then (d),
• then (b),
• and (c) diazotises most easily.

3. Matching with options

Checking the options:

• A: $(a)<(b)<(c)<(d)$
• B: $(a)<(d)<(b)<(c)$
• C: $(a)<(d)<(c)<(b)$
• D: $(d)<(c)<(b)<(a)$

The correct match is:

4. Final answer

The increasing order of diazotisation is: