To solve this question, the user needs to know the basics of Java String operations.
Option A: s3=s1 + s2
This is a legal operation. The + operator can be used to concatenate two strings. In this case, s1 and s2 are concatenated and the result is stored in s3.
Option B: s3=s1 - s2
This is an illegal operation. The - operator is not defined for strings in Java.
Option C: s3=s1 & s2
This is an illegal operation. The & operator is a bitwise operator and cannot be used with strings in Java.
Option D: s3=s1 && s2
This is an illegal operation. The && operator is a logical operator and cannot be used with strings in Java.