Among the following elements, which one is well known as a naturally radioactive element discovered by Marie and Pierre Curie?

Introduction / Context:
Radioactivity is a fundamental concept in nuclear chemistry and physics. Some elements are naturally radioactive, emitting radiation due to unstable nuclei. Famous discoveries in this field were made by Marie and Pierre Curie, who identified several radioactive elements. Many general knowledge and chemistry questions ask you to recall which elements are particularly associated with natural radioactivity and with the work of the Curies. Polonium is one of these historically significant elements.

Given Data / Assumptions:

• The elements listed are sulphur, polonium, tellurium and selenium.
• We are asked which is a radioactive element, not merely an element with radioactive isotopes.
• We assume a general knowledge context where polonium is highlighted as a radioactive element.
• The historical discovery by the Curies is relevant to the question.

Concept / Approach:
Polonium is a highly radioactive element discovered by Marie and Pierre Curie during their investigations of uranium ores. It has no stable isotopes and is known for intense alpha emission. Sulphur, tellurium and selenium, although they may have isotopes with different stabilities, are not typically cited as radioactive elements in introductory courses. When exam questions list polonium alongside ordinary non radioactive elements, the intention is usually to select polonium as the radioactive one.

Step-by-Step Solution:
Step 1: Recall that polonium, along with radium, was discovered by the Curies in their work on radioactivity.Step 2: Recognise that polonium has no stable isotopes and is strongly radioactive, primarily emitting alpha particles.Step 3: Consider sulphur, tellurium and selenium. These elements are generally treated as non radioactive in basic chemistry courses.Step 4: Match this information with the options and identify polonium as the element specifically known for being radioactive.Step 5: Therefore, select polonium as the correct answer.

Verification / Alternative check:
Historical accounts of the discovery of radioactivity often list uranium, thorium, polonium and radium as key radioactive elements. Polonium is named after Poland, the homeland of Marie Curie. It features in many examples concerning radioactivity in general science textbooks. Sulphur, selenium and tellurium are discussed mainly for their chemical properties and uses, not as examples of radioactivity. This historical and educational emphasis confirms that polonium is the intended answer.

Why Other Options Are Wrong:
Option a, sulphur, is a common non metal used in industry and has stable isotopes; it is not described as a radioactive element at the high school level. Option c, tellurium, and option d, selenium, are chalcogen elements with interesting semiconductor properties, but again they are not typically highlighted as radioactive in basic courses. Therefore, these options do not match the description of a well known radioactive element discovered by the Curies.

Common Pitfalls:
Students sometimes overthink the question and recall that many elements have some radioactive isotopes, leading them to doubt the obvious choice. However, in exam settings, "radioactive element" usually refers to elements like uranium, thorium, radium and polonium where radioactivity is a dominant property. Another pitfall is mixing up polonium with other p block elements because of their similar sounding names. To avoid this, remember the strong historical link: polonium was named by Marie Curie and is a classic example of a naturally radioactive element.

Final Answer:
The radioactive element among the options is polonium.