Assertion (A) : 2xyz + 3x² ia a cubic polynomial in three variables.

Reason (R) : An algebraic expression having two or more variables, the highest sum of the powers of all the variables in each term is taken as the degree of the polynomial, if this sum is a whole number.

1. Both A and R are correct, and R is the correct explanation for A.

2. Both A and R are correct, and R is not the correct explanation for A.

3. A is true, but R is false.

4. A is false, but R is true.

An algebraic expression having two or more variables, the highest sum of the powers of all the variables in each term taken as the degree of the polynomial if this sum is a whole number.

The maximum of these sums gives the degree. These powers must be whole numbers for it to be a valid polynomial.

So, reason (R) is true.

Given; 2xyz + 3x²

Terms; 2xyz, 3x²

For term 2xyz, degree = 1 + 1 + 1 = 3

For term 3x², degree = 2

The degree of a polynomial in multiple variables is the highest total degree among its terms.

Thus, the degree of this polynomial is 3.

Therefore, this is a cubic polynomial in three variables.

So, assertion (A) is true and, reason (R) is the correct explanation of assertion (A).

Hence, option 1 is the correct option.