Question

Complete the sentence with a suitable option:
Are you feeling doubtful ____ your decision?

Answer 1

Hint: Noun phrases are usually nouns with modifiers. Modifiers comes after a noun. They can include prepositional phrase, adjective clause, participle phrase, and infinitives. Most commonly used are prepositional phrases.

Complete answer:
In the above sentence, the words 'your decision' which follow the blank are in a noun phrase. Therefore, in the given phrase 'doubtful ______ your decision', the blank given is needed to be filled with the prepositions only, namely, 'about' or 'of' out of the four options.
The most common sentences that are often used are 'doubtful about something', or 'doubtful of something'.
The phrases using options B, C and D would be: 'doubt upon something', 'doubtful at something' and 'doubtful for something'. All these sentences are grammatically incorrect.
Thus, options B, C and D are incorrect.So, the correct answer is option A.
Usage of all options-
About: It is used 'on the subject of' something or 'concerning of' something or someone.
Upon: It is much more formal than on, but is used as the preposition for: on/onto an object or surface
At: It is used for a point in for an enclosed space or on for a surface.
For: It is used for indefinite periods of time, like "for the weekend", "for ages" or "for a long time".

Note:
A prepositional phrase in a sentence is a group of words containing a preposition, its object, and a word to modify that object. Usually, a prepositional phrase modifies a verb or a noun. The object used can be a noun, a gerund or a clause.