Acidic hydrolysis of ethyl isocyanide results in the formation of ethylamine salt and methanoic acid. Here's a detailed explanation of the process:

1. Definition of Ethyl Isocyanide:
- Ethyl isocyanide is an organic compound with the chemical formula C2H5NC. It consists of an ethyl group (C2H5) attached to a nitrogen atom (N) with a triple bond to a carbon atom (C).

2. Acidic Hydrolysis:
- Acidic hydrolysis is a chemical reaction in which a compound reacts with water under acidic conditions to produce two or more products.
- In the case of ethyl isocyanide, when it undergoes acidic hydrolysis, it reacts with water (H2O) in the presence of an acid catalyst.

3. Reaction Mechanism:
- The acidic hydrolysis of ethyl isocyanide proceeds through a nucleophilic attack by water on the isocyanide carbon atom.
- The nitrogen atom in ethyl isocyanide is relatively electron deficient due to the presence of the triple bond, making it susceptible to attack by nucleophiles.
- The water molecule acts as a nucleophile and attacks the carbon atom, breaking the triple bond and forming a new bond with the carbon atom.
- Simultaneously, the nitrogen atom forms a bond with a hydrogen ion (H+) from the acid catalyst to maintain its positive charge.
- As a result, the ethyl group is released as an ethylamine cation (C2H5NH3+) and the carbon atom is now bonded to an oxygen atom from the water molecule.

4. Products of Acidic Hydrolysis:
- The products of acidic hydrolysis of ethyl isocyanide are ethylamine salt and methanoic acid.
- The ethylamine cation (C2H5NH3+) combines with an anion (negative ion) from the acid catalyst, such as chloride (Cl-) or acetate (CH3COO-), to form the ethylamine salt.
- The carbon atom, now bonded to an oxygen atom, forms a carboxylic acid functional group, specifically methanoic acid (HCOOH), also known as formic acid.

Therefore, the correct answer is option 'A': Ethylamine salt and methanoic acid.