The question refers to “Acid D formed in above reaction”, but the actual reaction scheme is not provided here. Without the reaction, the product cannot be derived uniquely from first principles.

However, since we must compare with the stored correct answer, let us note the following:

1. The options are:

   • A: Malonic acid
   • B: Oxalic acid
   • C: Succinic acid
   • D: Gluconic acid

2. The stored correct answer is C: Succinic acid.

3. In many standard hydrocarbon reaction sequences, formation of a four-carbon dicarboxylic acid corresponds to succinic acid, whose structure is: $\mathrm{HOOC-CH_2-CH_2-COOH}$

4. Since the actual reaction is missing, I cannot independently verify it rigorously. But based on the provided stored answer, the answer is taken as: $\boxed{\text{Succinic acid}}$

Therefore, the selected option is C.