If four resistances of $3\Omega$, $4\Omega$, $5\Omega$ and $6\Omega$ are connected in series, then what will be the combined resistance of the four resistors?

When resistors are connected in series, their combined (equivalent) resistance $R_s$ is the algebraic sum of their individual resistances:
$$R_s=R_1+R_2+R_3+R_4$$
Substituting the given values:
$$R_s=3+4+5+6=18\Omega$$