In general science and geography, the intensity (effect on people and structures) of earthquakes is commonly measured on which of the following standard scales?

Introduction / Context:
Earthquakes are described using two different but related ideas: magnitude and intensity. Magnitude tells how much energy is released at the source, while intensity tells how strongly the shaking is felt at a particular place and how much damage it causes. This question specifically asks about the measurement of earthquake intensity, which is very important for disaster studies and competitive examinations.

Given Data / Assumptions:

• The question is about earthquake intensity, not magnitude at the epicentre.

• Several named scales are listed: Richter, Secant, Mercalli and Beaufort, plus one additional distractor.

• The learner is expected to know which scale is traditionally associated with intensity.

• Standard general knowledge convention is followed for the names of earthquake related scales.

Concept / Approach:
In basic seismology, the Richter scale (and related moment magnitude scales) are used to describe the magnitude of an earthquake, meaning the energy released at the focus. The Modified Mercalli Intensity (MMI) scale, on the other hand, describes the observed effects at the surface: shaking felt by people, movement of objects, and structural damage. Since the question uses the word intensity, the correct concept is the Mercalli type intensity scale and not the Richter scale.

Step-by-Step Solution:
Step 1: Carefully read the keyword intensity in the question. It refers to effects on the surface and on human structures. Step 2: Recall that Richter scale is used to measure magnitude, the energy released at the focus of the earthquake. Step 3: Remember that the Mercalli scale, especially the Modified Mercalli Intensity scale, ranks intensity from lower to higher levels based on damage and human perception. Step 4: Observe that Secant scale and Beaufort scale are not standard earthquake scales; Beaufort is used for wind. Step 5: Conclude that Mercalli scale is the correct answer for measuring earthquake intensity.

Verification / Alternative check:
Standard geography and general science textbooks usually present earthquake measurement through two separate table summaries: magnitude on the Richter or moment magnitude scale and intensity on the Mercalli or Modified Mercalli Intensity scale. Disaster management notes and seismology references repeat the same distinction. Checking any such reliable source confirms that Mercalli intensity values describe observed damage, while Richter values describe the energy of the event.

Why Other Options Are Wrong:
Richter scale is widely used for magnitude, not for intensity classification at different locations, so choosing it would mix up two related but different ideas. Secant scale is not used in earthquake measurement in basic general science. Beaufort scale measures wind force at sea and on land, not seismic events. The moment magnitude scale characterises the size of earthquakes similarly to Richter and again refers to magnitude rather than intensity. Therefore, all other options are incorrect for this specific question.

Common Pitfalls:
The most common mistake is to see the word earthquake and immediately pick Richter scale without reading the rest of the question. Another pitfall is not understanding the conceptual difference between magnitude and intensity. Candidates should train themselves to connect intensity with Mercalli and magnitude with Richter or moment magnitude. This simple association helps avoid frequent confusion in exam questions.

Final Answer:
The intensity of earthquakes is commonly measured on the Mercalli scale.