Explanation:

When Andrew approached, Susan was accompanied by her mother and a midwife.

Context:
The sentence mentions that Andrew approached Susan. This implies that Susan was in a situation where she was not alone, and someone was with her. The sentence further specifies that Susan was accompanied by her mother. It indicates that Susan's mother was present with her during this interaction.

Incomplete information:
The sentence leaves a blank space after mentioning Susan's mother and before the options (a), (b), (c), and (d). This blank space indicates that someone else was also present with Susan and her mother at that time.

Elimination:
Let's analyze the given answer options:

a) Joe: There is no prior mention of Joe or any other individual in the context. Therefore, this option can be eliminated.

b) Father: The sentence already mentions that Susan's mother was present. It does not provide any information about Susan's father. Thus, this option can also be eliminated.

c) A midwife: This option is supported by the context. A midwife is a healthcare professional who provides support to women during pregnancy, childbirth, and the postpartum period. In this scenario, the presence of a midwife suggests that Susan might be pregnant or in the process of giving birth.

d) Mother-in-law: There is no information given in the sentence that suggests Susan's marital status or the presence of her mother-in-law. Therefore, this option can be eliminated.

Conclusion:
Based on the elimination process and the information provided in the sentence, the correct answer is option (c) - a midwife.