Solution

Statement a:

Union with empty language gives the same language.

$\varphi + L = L$

So, statement a is true.

Statement b:

Concatenation with empty string does not change the language.

$\epsilon L = L\epsilon = L$

So, statement b is true.

Statement c:

Concatenation with empty language gives empty language.

$\varphi L = L\varphi = \varphi$

So, statement c is true.

Statement d:

This says concatenation with empty language gives $L$, which is false.

So, statement d is false.

Correct statements are:

a, b and c only