The correct option is D on charge
The option containing an error needs to be chosen.
From the underlined phrases, "on charge" has an error; it does not provide the correct meaning to the sentence. 'On charge' means when a device is charging its battery, then you say it's on charge. The correct phrase to be used instead is 'in charge', which means 'to be in control or overall responsibility'. It means that the baby is under the responsibility of the elderly lady. Hence B is the correct option as it has the error.
Option A: The phrase 'was left' has no error. (It's in the past tense, 'was' and 'left' both are in the past tense). Hence we do not choose option A.
Option C: 'An' is an indefinite article, used when the noun that follows is non-specific and begins with a vowel. (If there is an adjective that comes before the noun, then the adjective must begin with a vowel.) In this case, 'elderly', an adjective, begins with a vowel and the noun is not specific, hence the article 'an' has no error and we do not choose option C.
Option D: There isn't any error in "elderly lady", hence we do not choose this option.