Step 1: What decides boiling point.
All four are methyl halides with the same methyl group. So the boiling point depends on the strength of the van der Waals forces between molecules.
Step 2: What changes those forces.
These forces grow stronger as the halogen atom gets bigger and heavier, because bigger atoms have more electrons that can be pushed around.
Step 3: Order the halogens.
Going down group 17 the size and mass rise in the order $F < Cl < Br < I$.
Step 4: Order the boiling points.
So the boiling points follow the same trend, $\text{CH}_3\text{F} < \text{CH}_3\text{Cl} < \text{CH}_3\text{Br} < \text{CH}_3\text{I}$.
Step 5: Find the lowest.
Fluoromethane has the smallest, lightest halogen, so it has the weakest forces and the lowest boiling point.
Step 6: Final choice.
The lowest boiling point belongs to fluoromethane, which is option 1.
\[ \boxed{\text{Fluoromethane}} \]