The correct answer is: C. would be
The sentence “If only I were richer” is a conditional sentence of type 2. Conditional sentences of type 2 are used to talk about unreal or hypothetical situations. In this case, the speaker is expressing a wish that they were richer. The verb “were” is used in the past subjunctive mood, which is the verb form that is used in conditional sentences of type 2.
The other options are incorrect because they are not in the past subjunctive mood. Option A, “am,” is in the present tense. Option B, “were,” is also in the present tense, but it is the indicative mood, not the subjunctive mood. Option D, “will be,” is in the future tense.
Here is a table that summarizes the different verb forms that are used in conditional sentences:
| Type of Conditional Sentence | Verb Form |
| — | — |
| Type 1 | Present tense |
| Type 2 | Past subjunctive mood |
| Type 3 | Past perfect subjunctive mood |
I hope this explanation is helpful!