Metonymy is best defined as a figure of speech in which

Metonymy is best defined as a figure of speech in which one noun or attribute  is replaced by another that is closely associated with the replaced thing.

Explanation:

When a particular name or attribute or concept is replaced or substituted by another but the meaning remains the same, it is called as metonymy.

for example,

the use of the word "crown" to mean the whole kingdom under the authority of the king or queen, or power.

"society" meaning the people who live in a group etc.

1.c

2.a

3.d