In each of the following independent situations, indicate if the taxpayer must file Form 1040 in 2023:

a. Balaji is 27 years old, files as single, and has $8,400 of business income from his home-based web design business.

b. Jeremy, age 66, and Jenny, age 63, file married filing jointly and have income of $25,500.

c. Manxue, single and age 42, is blind and has income of $14,000.

d. Elaine, head of household and age 67, has income of $24,000.