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**Hint:**In this question, we have to find the value of $a$. So, the concept is to apply the basic logarithmic. The given expression, ${\log _a}\sqrt x = 4$ is in the form of identity ${\log _b}y = x$ and it can be written as ${b^x} = y$ form. And we also use the exponent law of power ${\left( {{y^m}} \right)^n} = {y^{m \times n}}$ to find the answer.

**Complete step by step answer:**

For finding the value of $a$, simplifying the given equation ${\log _a}\sqrt x = 4$.

The given expression is in the form of ${\log _b}y = k$ and it can be written as ${b^k} = y$

So, comparing the given equation from the identity we find that

$ \Rightarrow b = a$, $y = \sqrt x $ and $k = 4$

Therefore, the given equation can be written as

$ \Rightarrow {a^4} = \sqrt x $

We know that an equation can be raised to the same power on both sides without altering its value. Thus, raising the power of $\dfrac{1}{4}$ on both sides of the above equation, we’ll get:

\[ \Rightarrow {a^{\dfrac{4}{4}}} = {\left( {\sqrt x } \right)^{\dfrac{1}{4}}}\]

Square root means the power of $\dfrac{1}{2}$, putting this in the above equation, we’ll get

$ \Rightarrow a = {\left( {{x^{\dfrac{1}{2}}}} \right)^{\dfrac{1}{4}}}$

Further, from the exponent law of power we know that ${\left( {{y^m}} \right)^n} = {y^{m \times n}}$. Therefore we have:

$ \Rightarrow a = {x^{\dfrac{1}{2} \times \dfrac{1}{4}}} \\

\Rightarrow a = {x^{\dfrac{1}{8}}} \\

$

**Hence, option $\left( D \right)$ is correct.**

**Additional information:**

There are mainly two types of logarithm which we study, one is the logarithm of the base $10$ that is a common logarithm and the second is the logarithm of base $e$ that is a natural logarithm. We also study the logarithm of the base of any other whole number than $10$ and \[e\]. The logarithm of any negative number does not exist.

**Note:**

Some other properties of logarithm are:

$ \Rightarrow \log m + \log n = \log mn \\

\Rightarrow \log m - \log n = \log \dfrac{m}{n} \\

\Rightarrow a\log m = \log {m^a} \\ $

Logarithm problems are solved by frequently using these properties.

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