حَرْفُ اسْتِفْهَامٍ is a particle of questioning ( أ ) used at the beginning of a sentence.
َأ (is) + هَذَا (this is) = أَ هَذَا (Is this .... ?)
Examples :-
1) أَ هَذَا بَيْتٌ ؟ (Is this a house?)
2) أَ هَذَا جَمَلٌ ؟ (Is this a camel?)
3) أَ هَذَا مُدَرِّسٌ ؟ (Is this a teacher?)
