The correct answer is: A. constant returns to scale.
A Cobb-Douglas production function is a mathematical function that describes the relationship between the inputs (capital and labor) and the output of a production process. The function is given by the following equation:
$Q = AK^aL^b$
where $Q$ is the output, $K$ is the capital, $L$ is the labor, $A$ is the productivity parameter, and $a$ and $b$ are the output elasticities of capital and labor, respectively.
The output elasticity of capital is the percentage change in output that results from a 1% change in capital, holding labor constant. The output elasticity of labor is the percentage change in output that results from a 1% change in labor, holding capital constant.
If the output elasticities of capital and labor are equal, then the production function exhibits constant returns to scale. This means that if all inputs are increased by a factor of $x$, then output will also increase by a factor of $x$.
In the case of the Cobb-Douglas production function, constant returns to scale occur when $a+b=1$. In the given example, $a=0.6$ and $b=0.3$, so $a+b=0.9$. Therefore, the production function exhibits constant returns to scale.
Increasing returns to scale occur when $a+b>1$. This means that if all inputs are increased by a factor of $x$, then output will increase by more than a factor of $x$.
Decreasing returns to scale occur when $a+b<1$. This means that if all inputs are increased by a factor of $x$, then output will increase by less than a factor of $x$.