Concept:
In the Patanjali Yoga Sutra, “Praṇava” refers to the sacred syllable “Om” (\(\mathrm{Om}\)). Patanjali explains that Om is the verbal expression (Vāchaka) or symbol representing Ishwara.
The relevant sutra states:
\[
\textit{Tasya vācakaḥ praṇavaḥ}
\]
Meaning:
\[
\text{``Praṇava (Om) is the expression of Ishwara.''}
\]
Thus, Praṇava is associated with Ishwara in Yoga philosophy.
Step 1: Understand the meaning of Praṇava.
Praṇava is another name for the sacred syllable:
\[
\mathrm{Om}
\]
It is considered the primordial sound and is used extensively in meditation and yogic practices.
Step 2: Identify whose expression Praṇava is.
According to Patanjali:
\[
\textit{Tasya vācakaḥ praṇavaḥ}
\]
Here:
\[
\text{Tasya} \rightarrow \text{Ishwara}
\]
and
\[
\text{Praṇava} \rightarrow \text{Om}
\]
Thus:
\[
\text{Praṇava is the Vāchaka (expression) of Ishwara.}
\]
Step 3: Analyze the given options.
Option (A): Ishwar
Correct. Praṇava (Om) is the verbal symbol or expression of Ishwara.
Option (B): Purusha
Purusha refers to pure consciousness but is not specifically identified with Praṇava in this sutra.
Option (C): Om
Praṇava itself means Om, not the entity of which it is the expression.
Option (D): Jeeva
Jeeva means the individual soul and is not the Vāchya referred to here.
Step 4: Identify the correct option.
Since Praṇava is described as the expression of Ishwara, the correct answer is:
\[
\boxed{\text{Ishwar}}
\]