Step 1: Analyse the Assertion (A).
The Assertion states that Giuseppe Mazzini was a revolutionary from Italy. This statement is correct. Giuseppe Mazzini (1805--1872) was one of the greatest Italian nationalist leaders and revolutionaries of the nineteenth century. He dedicated his entire life to the cause of Italian unity, independence and the establishment of a democratic republic. His nationalist ideas inspired thousands of young Italians to struggle against foreign domination and absolute monarchy. Therefore, the Assertion is true.
Step 2: Analyse the Reason (R).
The Reason states that he founded underground organisations. This statement is also correct. After being forced into exile, Mazzini established secret revolutionary organisations such as Young Italy in 1831 and later Young Europe. These underground organisations aimed to unite the people, spread nationalist ideas and organise revolutionary movements against conservative rulers. Hence, the Reason is also true.
Step 3: Relationship between Assertion and Reason.
The Reason directly explains why Giuseppe Mazzini is remembered as a revolutionary. He did not merely preach nationalism through speeches and writings but actively organised secret revolutionary societies to unite Italy. Through these underground organisations, he mobilised young people, spread nationalist ideology and challenged the existing monarchical system. Thus, his revolutionary activities justified his identity as one of Italy's greatest revolutionaries.
Step 4: Historical significance of Mazzini's organisations.
The organisation Young Italy encouraged the youth to dedicate themselves to the cause of national unity and republicanism. Although many of Mazzini's early revolts were unsuccessful, his ideas inspired later leaders such as Count Cavour, Giuseppe Garibaldi and Victor Emmanuel II, who eventually completed the unification of Italy. Therefore, the establishment of underground organisations became one of his most important revolutionary contributions.
Step 5: Eliminate the incorrect options.
Option (B) is incorrect because the Reason does explain the Assertion. Option (C) is incorrect because the Reason is not false. Option (D) is also incorrect because both the Assertion and the Reason are historically correct. Therefore, only Option (A) satisfies both conditions of the Assertion--Reason question.
Step 6: Final conclusion.
Both the Assertion and the Reason are true, and the Reason correctly explains why Giuseppe Mazzini is regarded as a revolutionary from Italy. Hence, the correct answer is:
\[
\boxed{\textbf{(A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of Assertion (A).}}
\]
(A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of Assertion (A).