Para transformar a fala de Martha em discurso indireto (Reported Speech), precisamos considerar algumas regras gramaticais, como a mudança de tempos verbais e pronomes. A frase original de Martha é: "I don’t know what Peter is doing but you can come and stay at my place when you are in New York." 1. O verbo "don’t know" no passado se torna "didn’t know". 2. O presente contínuo "is doing" se transforma em "was doing". 3. O pronome "you" se torna "I" (porque a fala é sobre a pessoa que está reportando). 4. O pronome "my" se torna "her" (referindo-se a Martha). 5. O "can" se transforma em "could" no passado. 6. O "are" se transforma em "were". Analisando as alternativas: a) Martha said that she didn’t know what Peter was doing but I could come and stay at her place when I was in New York. (Correta, pois faz todas as mudanças necessárias.) b) Martha said that she doesn’t know what Peter is doing but you can come and stay at my place when you are in New York. (Incorreta, pois não faz a mudança de tempo.) c) Martha said that she did not know what Peter is doing but she can come and stayed at my place when she is in New York. (Incorreta, mistura tempos e pronomes.) d) Martha said that she will not know what Peter is doing but you can come and stay at my place when you are in New York. (Incorreta, usa o futuro em vez do passado.) e) Martha said that she would not know what Peter will be doing but you could come and stay at her place when you are in New York. (Incorreta, mistura tempos e não faz as mudanças corretas.) Portanto, a alternativa correta é: a. Martha said that she didn’t know what Peter was doing but I could come and stay at her place when I was in New York.