Step 1: Use the relation between frequency and wavelength.
All electromagnetic waves travel at the same speed \(c\) in vacuum, and \(c = \nu\lambda\), so frequency \(\nu = c/\lambda\). This means the wave with the largest wavelength has the smallest frequency.
Step 2: Compare typical wavelengths of the options.
Microwaves have wavelengths of the order of millimetres to centimetres; ultraviolet rays are around \(10^{-8}\) m; X-rays around \(10^{-10}\) m; and \(\gamma\)-rays even shorter, around \(10^{-12}\) m or less.
Step 3: Identify the longest wavelength.
Clearly the microwaves have by far the longest wavelength among these four.
Step 4: Conclude.
Since \(\nu = c/\lambda\), the longest-wavelength wave has the lowest frequency, so microwaves are the electromagnetic wave of minimum frequency.
\[\boxed{\text{Microwaves}}\]