Is lactic acid optically inactive.

Hint: We have to know that one type of stereoisomerism is optical isomerism. Optical isomers have the tendency to rotate the light that has plane polarized property. We refer to this property as optical activity.

Complete answer:
We can say that when a ray of plane polarized light goes through a solution that shows the presence of few organic molecules, the plane of polarization is rotated by an angle alpha. Those organic compounds which exhibit this property are said to be optically active. When the organic compounds rotate the plane polarized in the clockwise direction, then we can use the term dextrorotatory and it is indicated by the (+) sign. When the organic compounds rotate the plane polarized in the anti-clockwise direction, then we can use the term levorotatory and it is indicated by the (-) sign.
Optical activity of lactic acid: In a lactic acid particle there is a deviated chiral carbon atom with four unique groups which lead to two spatial compounds (d-lactic acid l-lactic acid) which are really imposable mirror representations of one another.
The constructions of two enantiomers of lactic acid are as per the following:
D-lactic acid turns the plane of enraptured light towards right while L-lactic acid pivots the plane of spellbound light towards left, which can be distinguished by polarimeter. So, Lactic acid is an optically active compound. We can structures of D-Lactic acid and L-Lactic acid as follows:
No, lactic acid is not optically inactive.

Note:
We have to remember that the compound in which the carbon atom is connected to four different groups is known as chiral compound. Chiral compounds do not have a plane of symmetry whereas achiral compounds have a plane of symmetry. Plane of symmetry is the plane which separates an object into two similar halves.