Half-life is measured by
A. Geiger-Muller counter
B. Carbon dating
C. Spectroscopic method
D. Wilson-Cloud chamber

Hint: In the given question, we need to determine the technique through which half-life is measured. For this, first we will define the meaning and significance of half-life.

Complete step by step solution:
We know that the period of time needed for a quantity to decrease to half of its original value is known as its half-life. The term is frequently used in nuclear physics to indicate just how long stable atoms last or how quickly unstable atoms decay radioactively. So, half-life can be measured by the Geiger-Muller counter.

Gamma, beta, and alpha radiation can all be identified and measured using a Geiger counter (also known as a Geiger-Muller tube). It essentially consists of two electrodes enclosed in gas. Also, a Geiger-Muller tube which has possibly the lowest pressure ever—contains a noble gas (often helium or argon) in this instrument. If the gas becomes conducive upon the use of a reasonably high voltage, the electrical change would imply radiation. Alpha, beta, and gamma radiation can all be detected using it.

Therefore, the correct option is A.

Note: Many students may write only the technique through which half-life is measured. But it is essential to explain the technique as well. The Geiger counter has fundamental shortcoming such as a Geiger-Müller tube cannot distinguish between different radiation types since its output pulse always has the same magnitude (independent of the energy of the input radiation).