A gamma emitting isotope emits gamma rays, which are a form of electromagnetic radiation. In order to detect this, we require an instrument that responds to this radiation, not to the visible characteristics of the sample.

Option 1, a microscope, is used to magnify and visually inspect microscopic entities. It can`t detect radiation.

Option 2, a lead plate, can help block gamma radiation due to its heavy density, but can`t itself detect the presence of gamma radiation.

Option 4, a spectrophotometer, measures light absorption to analyze the chemical composition of a sample. But it doesn`t detect radiation.

Option 3, a scintillation counter, however, is designed to detect and measure ionizing radiation including gamma rays. It uses the property of certain materials to emit light when excited by radiation, thereby capable of confirming the presence of a gamma emitting isotope.

So, the correct answer is 3 - Scintillation counter.