Answer:
The correct option is 1.
Explanation:
If terms have same variables with same degree, then the terms called like terms.
In option 1,
The given expression is
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Using commutative property of multiplication.
(cd=dc)
Now 5cd and -3cd are like terms. Combined like terms.


Expression 1 can be simplify by combining like terms. Therefore option 1 is correct.
In option 2,3 and 4,
The given expressions are
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

In these expressions, there are no like terms. Therefore option 2, 3 and 4 is incorrect.