Which of these congruences has no solution?

Explanation

1. 01

   $ax \equiv b \pmod{n}$ has a solution exactly when $\gcd(a, n)$ divides $b$.

2. 02

   $\gcd(6, 9) = 3$ does not divide 4. The others: 3 divides 3; $\gcd(2, 9) = 1$; $\gcd(4, 10) = 2$ divides 6.