Protein storing plastid is

The correct answer is aleuroplast.

Plastids are double-membrane-bound organelles found in the cells of plants and algae. Based on their function and the pigments they contain, plastids are classified into different types: chloroplasts, chromoplasts, and leucoplasts.
Leucoplasts are colorless plastids primarily responsible for storing nutrients. Depending on the type of nutrient they store, leucoplasts are further categorized as follows:

1. Aleuroplasts (also known as proteinoplasts): These specialized leucoplasts are responsible for storing proteins. They are commonly found in seeds, such as maize grains.
2. Amyloplasts: These store starch (carbohydrates) and are found in tubers like potatoes.
3. Elaioplasts: These store lipids, fats, and oils, and are typically found in oilseeds.
4. Chromoplasts: These are colored plastids containing carotenoid pigments, giving color (yellow, orange, or red) to flowers and fruits.

Therefore, the specific plastid that stores protein is the aleuroplast.