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, so $6x \equiv 4 \pmod{9}$ has none: $6x$ only leaves 0, 3 or 6.